[mips] Make it impossible to have UnknownABI in CodeGen and Integrated Assembler.

Summary:
This removes the need to coerce UnknownABI to the default ABI (O32 for
MIPS32, N64 for MIPS64 [*]) in both MipsSubtarget and MipsAsmParser.

Clang has been updated to disable both possible default ABI's before enabling
the ABI it intends to use.

[*] N64 being the default for MIPS64 is not actually correct.
    However N32 is not fully implemented/tested yet.

Depends on: D2830

Reviewers: jacksprat, matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D2832
Differential Revision: http://llvm-reviews.chandlerc.com/D2846

llvm-svn: 201792
diff --git a/llvm/test/CodeGen/Mips/mips64intldst.ll b/llvm/test/CodeGen/Mips/mips64intldst.ll
index 0e310a8..62244f6 100644
--- a/llvm/test/CodeGen/Mips/mips64intldst.ll
+++ b/llvm/test/CodeGen/Mips/mips64intldst.ll
@@ -1,5 +1,5 @@
-; RUN: llc  < %s -march=mips64el -mcpu=mips64 -mattr=n64 | FileCheck %s -check-prefix=CHECK-N64
-; RUN: llc  < %s -march=mips64el -mcpu=mips64 -mattr=n32 | FileCheck %s -check-prefix=CHECK-N32
+; RUN: llc  < %s -march=mips64el -mcpu=mips64 -mattr=-n64,n64 | FileCheck %s -check-prefix=CHECK-N64
+; RUN: llc  < %s -march=mips64el -mcpu=mips64 -mattr=-n64,n32 | FileCheck %s -check-prefix=CHECK-N32
 
 @c = common global i8 0, align 4
 @s = common global i16 0, align 4