fix RISC-V LMUL variant adaptation

[anthony-zy]

Fix glibc adaptation for different LMUL settings on RISC-V.

Background:

• RVV vector functions showed inconsistent behavior across LMUL variants.
• Some code paths did not correctly handle LMUL changes, leading to
  incorrect results or degraded performance.

Changes:

• Adjust macros and implementation to properly support multiple LMUL
  configurations.
• Ensure correctness while keeping other architectures unaffected.