remove invalid workaround for expf (glibc), 2X speedup

there was a workaround for x86_64 for glibc bug (4years ago),
where expf was slower than exp, so exp was used as a workaround.

that is not true anymore, use expf()  and code is 2X faster.

See and test: https://bugzilla.redhat.com/show_bug.cgi?id=521190
add NTA_ASM flag to optimizations, wrap all assembly code with it

define a new optimization flag NTA_ASM that enables our hand written ASM code be used.
It's important for debug/porting, where the ASM might not be able to run
check for SSE suppurt on all platforms

using PIC compilant ASM