build: default CROSS=riscv64-elf + document toolchain variants in README

[TumRedSun]

Makefile default changed from riscv64-unknown-elf to riscv64-elf (mainstream bare-metal toolchain). README.md and README.ru.md updated to reflect new default and document when to override.

Summary by Sourcery

Adopt riscv64-elf as the default bare-metal toolchain and document alternatives for building OnyxBoot.

Enhancements:

• Use riscv64-elf as the default bare-metal cross-compiler prefix while documenting supported toolchain variants and override guidance.

Build:

• Update Makefile build commands to use the new default cross-compiler configuration.

Documentation:

• Update English and Russian quick-start and toolchain documentation to reflect the new default and explain when to select riscv64-unknown-elf.

[sourcery-ai]

Reviewer's Guide

Switches the default RISC-V cross-compiler prefix to riscv64-elf and updates documentation to describe the new default and when/how to override it, including clarifying supported toolchain variants.

File-Level Changes

Change	Details	Files
Change default cross-compiler prefix to riscv64-elf and document supported/unsupported toolchain variants.	Replace default CROSS from riscv64-unknown-elf to riscv64-elf in the Makefile. Add detailed comments in the Makefile explaining preferred bare-metal toolchain variants and warning against using Linux-targeted toolchains. Update README quick-start to use plain make with a comment indicating the default CROSS value. Clarify in README tool table that riscv64-elf-g++ is preferred while still allowing riscv64-unknown-elf-g++. Add a section in README explaining the Makefile default, mapping it to mainstream bare-metal toolchains, and showing how to override for Debian's gcc-riscv64-unknown-elf. Mirror all README changes in README.ru.md with equivalent Russian phrasing, including default description and override instructions.	Makefile README.md README.ru.md

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[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• The Makefile recipe lines for bootloader.elf, bootloader.bin, clean, test, and test-all are now indented with spaces instead of tabs, which will break make; these should remain tab-indented shell commands.
• Consider mirroring the new Makefile note about riscv64-linux-gnu- being unsuitable for bare-metal in the README toolchain section so users who don’t inspect the Makefile still see that guidance.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The Makefile recipe lines for `bootloader.elf`, `bootloader.bin`, `clean`, `test`, and `test-all` are now indented with spaces instead of tabs, which will break `make`; these should remain tab-indented shell commands.
- Consider mirroring the new Makefile note about `riscv64-linux-gnu-` being unsuitable for bare-metal in the README toolchain section so users who don’t inspect the Makefile still see that guidance.

## Individual Comments

### Comment 1
<location path="Makefile" line_range="30-39" />
<code_context>

 bootloader.elf: $(SRCS)
-	$(CC) $(CFLAGS) $^ $(LDFLAGS) -o $@
+        $(CC) $(CFLAGS) $^ $(LDFLAGS) -o $@

 bootloader.bin: bootloader.elf
-	$(OBJCOPY) -O binary $< $@
+        $(OBJCOPY) -O binary $< $@

 clean:
-	rm -f bootloader.elf bootloader.bin
+        rm -f bootloader.elf bootloader.bin

 test:
-	./test/run_qemu.sh
+        ./test/run_qemu.sh

 test-all:
-	./test/test_all.sh
+        ./test/test_all.sh

 .PHONY: all clean test test-all
</code_context>
<issue_to_address>
**issue (bug_risk):** Use real tab characters for Make recipes instead of spaces to avoid `make` errors.

The recipe lines for `bootloader.elf`, `bootloader.bin`, `clean`, `test`, and `test-all` are now indented with spaces instead of a literal tab, which will trigger `missing separator` errors in many `make` implementations and break the build. Please change these indentations back to actual tab characters to keep the Makefile functional.
</issue_to_address>

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[sourcery-ai]

issue (bug_risk): Use real tab characters for Make recipes instead of spaces to avoid make errors.

The recipe lines for bootloader.elf, bootloader.bin, clean, test, and test-all are now indented with spaces instead of a literal tab, which will trigger missing separator errors in many make implementations and break the build. Please change these indentations back to actual tab characters to keep the Makefile functional.