[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.

Author: nathanchance

clang/lib/Basic/Targets/LoongArch.h

@@ -36,6 +36,7 @@ class LLVM_LIBRARY_VISIBILITY LoongArchTargetInfo : public TargetInfo {
     LongDoubleWidth = 128;
     LongDoubleAlign = 128;
     LongDoubleFormat = &llvm::APFloat::IEEEquad();
+    MCountName = "_mcount";
     SuitableAlign = 128;
     WCharType = SignedInt;
     WIntType = UnsignedInt;

clang/test/CodeGen/mcount.c

@@ -7,6 +7,8 @@
 // RUN: %clang_cc1 -pg -triple x86_64-netbsd -emit-llvm -o - %s | FileCheck -check-prefixes=CHECK-DOUBLE-PREFIXED,NO-MCOUNT1 %s
 // RUN: %clang_cc1 -pg -triple arm-netbsd-eabi -emit-llvm -o - %s | FileCheck -check-prefixes=CHECK-DOUBLE-PREFIXED,NO-MCOUNT1 %s
 // RUN: %clang_cc1 -pg -triple aarch64-netbsd -emit-llvm -o - %s | FileCheck -check-prefixes=CHECK-DOUBLE-PREFIXED,NO-MCOUNT1 %s
+// RUN: %clang_cc1 -pg -triple loongarch32 -emit-llvm -o - %s | FileCheck -check-prefixes=CHECK-PREFIXED,NO-MCOUNT1 %s
+// RUN: %clang_cc1 -pg -triple loongarch64 -emit-llvm -o - %s | FileCheck -check-prefixes=CHECK-PREFIXED,NO-MCOUNT1 %s
 // RUN: %clang_cc1 -pg -triple mips-netbsd -emit-llvm -o - %s | FileCheck -check-prefixes=CHECK-DOUBLE-PREFIXED,NO-MCOUNT1 %s
 // RUN: %clang_cc1 -pg -triple mips-unknown-gnu-linux -emit-llvm -o - %s | FileCheck -check-prefixes=CHECK-PREFIXED,NO-MCOUNT1 %s
 // RUN: %clang_cc1 -pg -triple mipsel-unknown-gnu-linux -emit-llvm -o - %s | FileCheck -check-prefixes=CHECK-PREFIXED,NO-MCOUNT1 %s