Implement riscv_vlen_asm for riscv32

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Now for the linux32-riscv32 target, we would get the error when linking

riscvcap.c:(.text.startup+0x104): undefined reference to `riscv_vlen_asm'

#20149 added riscv_vlen_asm in riscv64cpuid.pl but not in riscv32cpuid.pl, and this function is needed in riscvcap.c.

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