add support for a prebuilt clang

ps2: RETAIN SUPPORT FOR STOCK CLANG... ASDLKJSHADLKSJAHDLKDHJA
ps3: derp
ps4: set stock clang
ps5: fixup the header inclusion
ps6: add guard for default clang version
ps7: cortex-15 if supported by both clang versions, and fixup header inclusion
ps8: fix derp
ps9: FIX SHIT UP FOR REAL?
ps10: copy/paste derp
ps11: ifeq--->ifneq
Change-Id: I07c1725bec1600d7082ff3b57d0b82820eb408a1

Conflicts:

	core/dumpvar.mk
	core/llvm_config.mk