Allow '@' to appear in x86 mingw symbols

[rnk]

Summary

There is no reason to differ in assembler behavior here between -msvc
and -gnu targets. Without this setting, the text after the '@' is
interpreted as a symbol variable, like foo@IMGREL.

[mstorsjo]

LGTM, thanks!

llvm/lib/Target/X86/MCTargetDesc/X86MCAsmInfo.cpp

X86MCAsmInfoGNUCOFF::X86MCAsmInfoGNUCOFF(const Triple &Triple) {
   } else {
     ExceptionsType = ExceptionHandling::DwarfCFI;
   }
 
   AssemblerDialect = AsmWriterFlavor;
 
   TextAlignFillValue = 0x90;
 
+  AllowAtInName = true;
+
   UseIntegratedAssembler = true;
 }

llvm/test/MC/X86/stdcall.s

+# RUN: llvm-mc %s -filetype=obj -o - -triple i686-windows-msvc | llvm-nm - | FileCheck %s
+# RUN: llvm-mc %s -filetype=obj -o - -triple i686-windows-gnu | llvm-nm - | FileCheck %s
+
+# CHECK: T _mystdcall@8{{$}}
+# CHECK: T foo
+
+.text
+.global _mystdcall@8
+_mystdcall@8:
+  movl 4(%esp), %eax
+  addl 8(%esp), %eax
+  retl $8
+
+.global foo
+foo:
+  pushl $1
+  pushl $2
+  calll _mystdcall@8
+  retl