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/ARM/ARMAsmPrinter.cpp b/llvm/lib/Target/ARM/ARMAsmPrinter.cpp
index b4e0791..49c2512 100644
--- a/llvm/lib/Target/ARM/ARMAsmPrinter.cpp
+++ b/llvm/lib/Target/ARM/ARMAsmPrinter.cpp
@@ -419,7 +419,7 @@
 }
 
 static bool isThumb(const MCSubtargetInfo& STI) {
-  return (STI.getFeatureBits() & ARM::ModeThumb) != 0;
+  return STI.getFeatureBits()[ARM::ModeThumb];
 }
 
 void ARMAsmPrinter::emitInlineAsmEnd(const MCSubtargetInfo &StartInfo,