Use std::bitset for SubtargetFeatures

Previously, subtarget features were a bitfield with the underlying type being uint64_t. 
Since several targets (X86 and ARM, in particular) have hit or were very close to hitting this bound, switching the features to use a bitset.
No functional change.

The first two times this was committed (r229831, r233055), it caused several buildbot failures. 
At least some of the ARM and MIPS ones were due to gcc/binutils issues, and should now be fixed.

llvm-svn: 237234
diff --git a/llvm/lib/Target/Mips/MCTargetDesc/MipsMCCodeEmitter.cpp b/llvm/lib/Target/Mips/MCTargetDesc/MipsMCCodeEmitter.cpp
index 0cb8d55..256cdfd 100644
--- a/llvm/lib/Target/Mips/MCTargetDesc/MipsMCCodeEmitter.cpp
+++ b/llvm/lib/Target/Mips/MCTargetDesc/MipsMCCodeEmitter.cpp
@@ -112,11 +112,11 @@
 }
 
 bool MipsMCCodeEmitter::isMicroMips(const MCSubtargetInfo &STI) const {
-  return STI.getFeatureBits() & Mips::FeatureMicroMips;
+  return STI.getFeatureBits()[Mips::FeatureMicroMips];
 }
 
 bool MipsMCCodeEmitter::isMips32r6(const MCSubtargetInfo &STI) const {
-  return STI.getFeatureBits() & Mips::FeatureMips32r6;
+  return STI.getFeatureBits()[Mips::FeatureMips32r6];
 }
 
 void MipsMCCodeEmitter::EmitByte(unsigned char C, raw_ostream &OS) const {