Beware of GCC 5.x

[vadimcn]

Just a heads-up to others: mingw-w64 (not sure about other platforms) GCC 5.x appears to be subtly mis-compiling LLVM, which results in a segmentation fault during rustc's codegen.
I've seen this with x86_64-5.1.0- and i686-5.2.0- toolchains. In the former case LLVM had to be compiled with asserts (--enable-llvm-assertions) for this to occur.

GCC 4.9.3 seems to be OK.

[alexcrichton]

Closing as a dupe of #28260