Allow memset local PLT reference for RISC-V.

This is similar to commit a26e2e9f
"Allow memset local PLT reference for powerpc soft-float.".

GCC 10.1 results in the localplt test failing for RISC-V.

From the original commit for power-pc:
    Since memset is documented as a function GCC may always implicitly
    generate calls to, it seems reasonable to allow that local PLT
    reference (just like those for libgcc functions that GCC implicitly
    generates calls to and that are also exported from libc.so), which
    this patch does.

Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>