x86: Experimental nasm support
Enables the use of nasm as an alternative to yasm. Note that nasm cannot assemble x264 with PIC enabled since it currently doesn't support [symbol-$$] addressing which is used extensively by x264's PIC code. This includes all 64-bit Windows and 64-bit OS X builds, even non-shared. For the above reason nasm is currently intentionally not auto-detected, instead the assembler must be explicitly specified using "AS=nasm ./configure". Also drop -O2 from ASFLAGS since it's simply ignored anyway.