runtime: add ClearSpan experiment to clear spans on first allocation

[lxq015]

Port of https://gitcode.com/openeuler/golang/pull/61 to the ZTE RISC-V toolchain.

goos: linux
goarch: riscv64
pkg: runtime
cpu: Spacemit(R) X60

                  │ bench_malloc_clearspan_before.txt │ bench_malloc_clearspan_after.txt │
                  │                      sec/op                      │           sec/op             vs base               │
Malloc8                                                  101.4n ± 6%                   103.6n ± 4%        ~ (p=0.290 n=6)
Malloc16                                                 180.8n ± 1%                   145.2n ± 3%  -19.64% (p=0.002 n=6)
Malloc32                                                 218.4n ± 0%                   172.8n ± 0%  -20.90% (p=0.002 n=6)
MallocTypeInfo8                                          151.6n ± 2%                   128.7n ± 2%  -15.11% (p=0.002 n=6)
MallocTypeInfo16                                         205.8n ± 8%                   183.3n ± 1%  -10.95% (p=0.002 n=6)
MallocTypeInfo32                                         244.6n ± 1%                   210.9n ± 1%  -13.78% (p=0.002 n=6)
MallocLargeStruct                                        1.432µ ± 1%                   1.385µ ± 1%   -3.25% (p=0.002 n=6)
geomean                                                  238.4n                        209.8n       -11.98%

This change introduces the ClearSpan GOEXPERIMENT, which clears an entire span when the first object is allocated from a span that still requires zeroing.

The ClearSpan path moves zeroing work from each individual small allocation to the first allocation from the span. Once the span is cleared, needzero is reset so later allocations from the same span can avoid repeated per-object clearing. Tiny allocation zeroing is also adjusted to avoid redundant clearing when the span has already been zeroed.

To enable this experiment, build the Go compiler with GOEXPERIMENT=tinysize.

Tests are added to validate that ClearSpan resets needzero and clears the backing memory only when the experiment is enabled.

Authored-by: cyf_123123

[wangpc-pp]

+2

[ctk-1998]

+1

[wenchangping]

+1