RISC-V: lib: Add pi aliases for string functions

RISC-V: lib: Add pi aliases for string functions

memset, strcmp, and strncmp are all used in the __pi_ section,
add SYM_FUNC_ALIAS for them.

When KASAN is enabled in <asm/string.h> __pi___memset is also needed.

Suggested-by: Charlie Jenkins <charlie@rivosinc.com>
Signed-off-by: Jesse Taube <jesse@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240709173937.510084-3-jesse@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>