libs/libc/risc-v: Refresh memcpy and memset with XLEN-adaptive loops.

[Shanks0224]

libs/libc/risc-v: Refresh memcpy and memset with XLEN-adaptive loops.

Summary

Rewrite arch_memcpy.S and arch_memset.S to be register-width aware on
both RV32 and RV64 using REG_L/REG_S/SZREG macros from asm.h.

The old memcpy always used lw/sw even on RV64, wasting half the memory
bandwidth, and unrolled only 64 bytes per iteration. The old memset
unrolled only 16 bytes per iteration.

Changes:

• memcpy: 16xSZREG unrolled main loop (128B/iter on RV64, 64B on RV32),
  shift-merge path for misaligned source so no load or store is ever
  unaligned, single SZREG and byte loops for remainder.
• memset: 32xSZREG unrolled main loop (256B/iter on RV64, 128B on RV32)
  with Duff's device for non-power-of-two remainders. .option norvc
  ensures fixed 4-byte instruction width for correct jump calculation.
  Zero-length input handled correctly.

Impact

• Is new feature added? NO. Existing functions refreshed.
• Impact on user? NO.
• Impact on build? NO.
• Impact on hardware? NO.
• Impact on documentation? NO.
• Impact on security? NO.
• Impact on compatibility? NO. Only active when CONFIG_RISCV_STRING_FUNCTION=y (default n).

Testing

I confirm that changes are verified on local setup and works as intended:

• Build Host: Linux x86_64, riscv-none-elf-gcc 13.2.1
• Target(s): QEMU rv-virt RV32 (rv-virt:nsh), QEMU rv-virt RV64 (rv-virt:nsh64)

Correctness: arch_libctest reports PASSED for memcpy and memset across
alignments 0-7 and sizes 1-128.

Performance (QEMU RV32, rdcycle, 128 bytes, 100 iterations average):

                     baseline    optimized    speedup
  memcpy(128)           952          676       1.41x
  memset(128)           562          237       2.37x

Testing logs (optimized):

nsh> arch_libctest
Testing memcpy...
memcpy: PASSED
memcpy(128) avg cycles: 676
Testing memset...
memset: PASSED
memset(128) avg cycles: 237
...
arch_libc_test Passed

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb

[xiaoxiang781216]

@Fishwaldo could you benchmark this patch on your hardware?