optimize: use fast asm code also on linux32 linux64

this code was allowed only for NTA_PLATFORM_darwin86,
but comment mentions it does not work only on win32, so adding linux for
speed up!
another 10x speedup for UNIX
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
Revert "optimize: use fast asm code also on linux32 linux64"

This reverts commit 0b7c792fb8d14f08c999ac7a3e7a9a7f07c80c27.
builds fine on my machine, Travis fails though
Revert "another 10x speedup for UNIX", assembly problems?

This reverts commit 0746cac8374cd394d51859ba5c1d3d057fa0cfe6.