Allow '@' to appear in x86 mingw symbols

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.

Reviewers: mstorsjo

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66974

llvm-svn: 370408