Always disable GlobalISel on aarch64, fixes a crash when building on
aarch64 without retguard, with a stack protector and without
optimizations.

Index: lib/Target/AArch64/AArch64TargetMachine.cpp
--- lib/Target/AArch64/AArch64TargetMachine.cpp.orig
+++ lib/Target/AArch64/AArch64TargetMachine.cpp
@@ -332,6 +332,7 @@ AArch64TargetMachine::AArch64TargetMachine(const Targe
   // Enable GlobalISel at or below EnableGlobalISelAt0, unless this is
   // MachO/CodeModel::Large, which GlobalISel does not support.
   if (getOptLevel() <= EnableGlobalISelAtO &&
+      !getTargetTriple().isOSOpenBSD() &&
       TT.getArch() != Triple::aarch64_32 &&
       TT.getEnvironment() != Triple::GNUILP32 &&
       !(getCodeModel() == CodeModel::Large && TT.isOSBinFormatMachO())) {