Removing support for libcxxrt.

libc++abi now seems to work fine on both Linux and Darwin, and it's
(mostly) integrated into the LLVM build system.
Fixing typo.

Note: doesn't compile on Linux anymore right now. Sigh.

/usr/bin/ld:
../../lib/libLLVMAArch64CodeGen.a(AArch64AsmPrinter.cpp.o): relocation
R_X86_64_32S against `.text' can not be used when making a shared
object; recompile with -fPIC