Add x86_64 target configure defaults

It includes:
-march=atom/-mtune=atom to focus on Atom instructions/optimizations;
-fpmath=sse to generate sse math instructions instead of i387
-m32/-m64/-mx32 support to be able to generate 32/64/x32 code with
single toolchain.

Change-Id: I1a650b4cf572322baa5648a97e1731922571a132
Signed-off-by: Pavel Chupin <pavel.v.chupin@intel.com>