add llvm configure flags to enable/disable target OSes/Envs

lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp

Index: lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp
===================================================================
--- lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp	(revision 138335)
+++ lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp	(working copy)
@@ -118,8 +118,7 @@
 
   if (TheTriple.isOSDarwin())
     return isEH ? DWARFFlavour::X86_32_DarwinEH : DWARFFlavour::X86_32_Generic;
-  if (TheTriple.getOS() == Triple::MinGW32 ||
-      TheTriple.getOS() == Triple::Cygwin)
+  if (TheTriple.isOSMinGW32() || TheTriple.isOSCygwin())
     // Unsupported by now, just quick fallback
     return DWARFFlavour::X86_32_Generic;
   return DWARFFlavour::X86_32_Generic;
@@ -289,7 +288,7 @@
   bool is64Bit = TheTriple.getArch() == Triple::x86_64;
 
   MCAsmInfo *MAI;
-  if (TheTriple.isOSDarwin() || TheTriple.getEnvironment() == Triple::MachO) {
+  if (TheTriple.isOSDarwin() || TheTriple.isEnvironmentMachO()) {
     if (is64Bit)
       MAI = new X86_64MCAsmInfoDarwin(TheTriple);
     else
@@ -374,7 +373,7 @@
                                     bool NoExecStack) {
   Triple TheTriple(TT);
 
-  if (TheTriple.isOSDarwin() || TheTriple.getEnvironment() == Triple::MachO)
+  if (TheTriple.isOSDarwin() || TheTriple.isEnvironmentMachO())
     return createMachOStreamer(Ctx, MAB, _OS, _Emitter, RelaxAll);
 
   if (TheTriple.isOSWindows())