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.