[RISCV] Fix mcount name

GCC's name for this symbol is _mcount, which the Linux kernel expects in
a few different place:

  $ echo 'int main(void) { return 0; }' | riscv32-linux-gcc -c -pg -o tmp.o -x c -

  $ llvm-objdump -dr tmp.o | grep mcount
                          0000000c:  R_RISCV_CALL _mcount

  $ echo 'int main(void) { return 0; }' | riscv64-linux-gcc -c -pg -o tmp.o -x c -

  $ llvm-objdump -dr tmp.o | grep mcount
                  000000000000000c:  R_RISCV_CALL _mcount

  $ echo 'int main(void) { return 0; }' | clang -c -pg -o tmp.o --target=riscv32-linux-gnu -x c -

  $ llvm-objdump -dr tmp.o | grep mcount
                          0000000a:  R_RISCV_CALL_PLT     mcount

  $ echo 'int main(void) { return 0; }' | clang -c -pg -o tmp.o --target=riscv64-linux-gnu -x c -

  $ llvm-objdump -dr tmp.o | grep mcount
                  000000000000000a:  R_RISCV_CALL_PLT     mcount

Set MCountName to "_mcount" in RISCVTargetInfo then prevent it from
getting overridden in certain OSTargetInfo constructors.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D98881



Signed-off-by: Nathan Chancellor <nathan@kernel.org>