Do not enable SSE3 globally

Instead of enabling SSE3 instructions globally, only use it for files
that specifically require those instructions. Skia will call that code
based on a runtime check of processor features. Instead enable SSE2
globally except for opts_check_SSE2.cpp which requires extensions to be
disabled to properly check processor features.

Also fix a typo in the detection of i686, TARTET -> TARGET.

Fixes #39.