Revert "use linaro toolchain"

This reverts commit d838df385f2623cb14cae6b150e37147ebdc6767.
Add TARGET_GCC_VERSION

Default to 4.6, but can overwritten to something else in
prebuilts/gcc/$os/$arch/$arch-linux-android*-$ver

Change-Id: I16f99d642b8ff8e831d392261a0c536041107051
core: config: use 4.7 by default unless TARGET_GCC_VERSION is set otherwise
envsetup: use 4.7 by default
core: more 4.7 fixes