[Clang][LoongArch] Generate _mcount instead of mcount (#65657)

When building the LoongArch Linux kernel without
`CONFIG_DYNAMIC_FTRACE`, the build fails to link because the mcount
symbol is `mcount`, not `_mcount` like GCC generates and the kernel
expects:

```
ld.lld: error: undefined symbol: mcount
>>> referenced by version.c
>>>               init/version.o:(early_hostname) in archive vmlinux.a
>>> referenced by do_mounts.c
>>>               init/do_mounts.o:(rootfs_init_fs_context) in archive vmlinux.a
>>> referenced by main.c
>>>               init/main.o:(__traceiter_initcall_level) in archive vmlinux.a
>>> referenced 97011 more times
>>> did you mean: _mcount
>>> defined in: vmlinux.a(arch/loongarch/kernel/mcount.o)
```

Set `MCountName` in `LoongArchTargetInfo` to `_mcount`, which resolves
the build failure.