Use TARGET_ARCH_VARIANT_CPU for cortex-a8 fix.

Not all devices set cpu=cortex-a9 for TARGET_EXTRA_CFLAGS, so use a
more common define to check when the cortex-a8 fix should be applied

Change-Id: I93c9c4e429e5cb685f6540b493656ae3a4c7e5ad
Remove extranneous export -f lines

uneccesary when calling source build/envsetup.sh and causes extra output in zsh

Taken from: http://review.cyanogenmod.org/#/c/44993/

Change-Id: Ib5bda44b9963b4dc1ab434750cbaccc5c62eac10
Support building with GNU make 4.0

* It works just fine, no need for the warning.

Change-Id: Ifa2ab17454edfcd8b7840a7534f70576286f2864