build: add support for a prebuilt clang

- set TARGET_CLANG_VERSION to your desired clang version to use it

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
ps12: don't enable by default
ps14: revise conditionalization if CLANG_EXTRA_INCLUDES and restore mcpu substitution for global stock clang
Change-Id: I07c1725bec1600d7082ff3b57d0b82820eb408a1
Signed-off-by: bart452 <bart4524@gmail.com>

Conflicts:
	core/clear_vars.mk
	core/dumpvar.mk
Prebuilt clang: add a TARGET_ENABLE_PREBUILT_CLANG flag, which sets TARGET_CLANG_VERSION to msm-3.4

simply setting TARGET_CLANG_VERSION still enables use of prebuilt clang, as before this commit.

Change-Id: Ib8cb6312d9a7f9ffff3e48c5c66bfb80893bcf97