libc++abi: add a top level option for using CompilerRT

Add an option to opt into compiler-rt instead of libgcc.  This option defaults
to OFF to avoid a behaviour change.  It is not possible to mix and match
different runtime libraries.  Disabling this requires that libc++ is built
accordingly.  This knob is particularly useful for targets that are GCC by
default (i.e. Linux).

llvm-svn: 275505