loongarch: use "_mcount" as the default mcount symbol

Set the `mcount` field to "_mcount" for all LoongArch targets to match
Clang/GCC behavior [1]. This fixes linking failures when using
`-Z instrument-mcount`, where the runtime expects "_mcount" instead of
"mcount".

[1] https://github.com/llvm/llvm-project/blob/main/clang/lib/Basic/Targets/LoongArch.h#L60

Author: heiher

compiler/rustc_target/src/spec/targets/loongarch32_unknown_none.rs

@@ -22,6 +22,7 @@ pub(crate) fn target() -> Target {
             linker: Some("rust-lld".into()),
             llvm_abiname: LlvmAbi::Ilp32d,
             max_atomic_width: Some(32),
+            mcount: "_mcount".into(),
             relocation_model: RelocModel::Static,
             panic_strategy: PanicStrategy::Abort,
             ..Default::default()

compiler/rustc_target/src/spec/targets/loongarch32_unknown_none_softfloat.rs

@@ -23,6 +23,7 @@ pub(crate) fn target() -> Target {
             linker: Some("rust-lld".into()),
             llvm_abiname: LlvmAbi::Ilp32s,
             max_atomic_width: Some(32),
+            mcount: "_mcount".into(),
             relocation_model: RelocModel::Static,
             panic_strategy: PanicStrategy::Abort,
             ..Default::default()

compiler/rustc_target/src/spec/targets/loongarch64_unknown_linux_gnu.rs

@@ -20,6 +20,7 @@ pub(crate) fn target() -> Target {
             features: "+f,+d,+lsx,+relax".into(),
             llvm_abiname: LlvmAbi::Lp64d,
             max_atomic_width: Some(64),
+            mcount: "_mcount".into(),
             supported_sanitizers: SanitizerSet::ADDRESS
                 | SanitizerSet::CFI
                 | SanitizerSet::LEAK

compiler/rustc_target/src/spec/targets/loongarch64_unknown_linux_musl.rs

@@ -20,6 +20,7 @@ pub(crate) fn target() -> Target {
             features: "+f,+d,+lsx,+relax".into(),
             llvm_abiname: LlvmAbi::Lp64d,
             max_atomic_width: Some(64),
+            mcount: "_mcount".into(),
             crt_static_default: false,
             supported_sanitizers: SanitizerSet::ADDRESS
                 | SanitizerSet::CFI

compiler/rustc_target/src/spec/targets/loongarch64_unknown_linux_ohos.rs

@@ -20,6 +20,7 @@ pub(crate) fn target() -> Target {
             features: "+f,+d,+lsx,+relax".into(),
             llvm_abiname: LlvmAbi::Lp64d,
             max_atomic_width: Some(64),
+            mcount: "_mcount".into(),
             supported_sanitizers: SanitizerSet::ADDRESS
                 | SanitizerSet::CFI
                 | SanitizerSet::LEAK

compiler/rustc_target/src/spec/targets/loongarch64_unknown_none.rs

@@ -22,6 +22,7 @@ pub(crate) fn target() -> Target {
             linker: Some("rust-lld".into()),
             llvm_abiname: LlvmAbi::Lp64d,
             max_atomic_width: Some(64),
+            mcount: "_mcount".into(),
             relocation_model: RelocModel::Static,
             panic_strategy: PanicStrategy::Abort,
             code_model: Some(CodeModel::Medium),

compiler/rustc_target/src/spec/targets/loongarch64_unknown_none_softfloat.rs

@@ -23,6 +23,7 @@ pub(crate) fn target() -> Target {
             linker: Some("rust-lld".into()),
             llvm_abiname: LlvmAbi::Lp64s,
             max_atomic_width: Some(64),
+            mcount: "_mcount".into(),
             relocation_model: RelocModel::Static,
             panic_strategy: PanicStrategy::Abort,
             code_model: Some(CodeModel::Medium),