scripts/recordmcount.pl: Fix RISC-V regex for clang

Clang can generate R_RISCV_CALL_PLT relocations to _mcount:

$ llvm-objdump -dr build/riscv/init/main.o | rg mcount
                000000000000000e:  R_RISCV_CALL_PLT     _mcount
                000000000000004e:  R_RISCV_CALL_PLT     _mcount

After this, the __start_mcount_loc section is properly generated and
function tracing still works.

Link: https://github.com/ClangBuiltLinux/linux/issues/1331


Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Fangrui Song <maskray@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>