Use the existing build configuration parameter ENABLE_BACKTRACE to compile out...

Use the existing build configuration parameter ENABLE_BACKTRACE to compile out all pretty stack trace support when backtraces are disabled.

This has the nice secondary effect of allowing LLVM to continue to build
for targets without __thread or thread_local support to continue to work
so long as they build without support for backtraces.

llvm-svn: 227423