Fix mcount name for *-windows-gnu targets

mingw exposes the `_mcount` symbol for profiling.

Author: pmur

compiler/rustc_target/src/spec/base/windows_gnu.rs

@@ -106,6 +106,7 @@ pub(crate) fn opts() -> TargetOptions {
         // FIXME(davidtwco): Support Split DWARF on Windows GNU - may require LLVM changes to
         // output DWO, despite using DWARF, doesn't use ELF..
         supported_split_debuginfo: Cow::Borrowed(&[SplitDebuginfo::Off]),
+        mcount: "_mcount".into(),
         ..Default::default()
     }
 }

compiler/rustc_target/src/spec/base/windows_gnullvm.rs

@@ -53,6 +53,7 @@ pub(crate) fn opts() -> TargetOptions {
         // FIXME(davidtwco): Support Split DWARF on Windows GNU - may require LLVM changes to
         // output DWO, despite using DWARF, doesn't use ELF..
         supported_split_debuginfo: Cow::Borrowed(&[SplitDebuginfo::Off]),
+        mcount: "_mcount".into(),
         ..Default::default()
     }
 }