Disable indirect symbols via GOT pcrel on Mach-O 64-bit

This commit currently causes LLVM to segfault when compiling Rust code on 64-bit
OSX platforms, reported as this LLVM bug:

    https://llvm.org/bugs/show_bug.cgi?id=23779

A bisection turned up r230264 as the offending commit, the review of which is at
http://reviews.llvm.org/D6922. To the best of my knowledge this looks like it's
an optimization for OSX which is fine to omit, so this commit just flips the
flag to *not* perform this optimization, allowing Rust to bootstrap and pass all
tests on OSX 64-bit.

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -437,7 +437,7 @@ TargetLoweringObjectFileELF::InitializeELF(bool UseInitArray_) {
 
 TargetLoweringObjectFileMachO::TargetLoweringObjectFileMachO()
   : TargetLoweringObjectFile() {
-  SupportIndirectSymViaGOTPCRel = true;
+  SupportIndirectSymViaGOTPCRel = false;
 }
 
 /// getDepLibFromLinkerOpt - Extract the dependent library name from a linker