[mips] Set GCC-compatible MIPS asssembler options before inline asm blocks.

Summary:
When generating MIPS assembly, LLVM always overrides the default assembler options by emitting the '.set noreorder', '.set nomacro' and '.set noat' directives,
while GCC uses the default options if an assembly-level function contains inline assembly code.

This becomes a problem when the code generated by LLVM is interleaved with inline assembly which assumes GCC-like assembler options (from Linux, for example).

This patch fixes these conflicts by setting the appropriate assembler options at the beginning of an inline asm block and popping them at the end.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6637

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224425 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/AsmPrinter.h

@@ -469,6 +469,10 @@ public:
                                      unsigned AsmVariant, const char *ExtraCode,
                                      raw_ostream &OS);
 
+  /// Let the target do anything it needs to do before emitting inlineasm.
+  /// \p StartInfo - the subtarget info before parsing inline asm
+  virtual void emitInlineAsmStart(const MCSubtargetInfo &StartInfo) const;
+
   /// Let the target do anything it needs to do after emitting inlineasm.
   /// This callback can be used restore the original mode in case the
   /// inlineasm contains directives to switch modes.