Copy SOURCES to on-device toolchain directory for version control

Also add more flags to reduce size of ld.mcld to [more than 10 times]
compensate for the loss of space to the new SOURCES file

Change-Id: I9c24553482a1aefbef7f0d75fbd473fa19bb0130
Add Script for building abcc.

Change-Id: I0a7c73a1021dd501abf5ec50a53f0d00778565d3
Enable parallel llvm code-gen

Change-Id: I963b11769921d7f235aa1b196665e2ef0e083083
Refine abcc for system image.

After apply this patch:
1. Use build-abcc.sh with --only-assets to generate toolchain assets.
3. mmm ndk/tests/abcc
4. make systemimage

Also 2 modifications.
1. Change name abcc-$abi.apk to abcc-$abi-unsigned.apk for clearity.
2. Add prebuilts dir into .gitignore.

Change-Id: Iadace270113209e958439250e6db75c9ed6ea0b8

Conflicts:
	tests/abcc/jni/Android.mk