Cleanup x86 and x86_64 flags

-mandroid, -D__ANDROID__ is on by the toolchain;
-msse3 moved to llvm_config, it seems it's required only there;
__cxa_atexit is provided by libc now;
USE_SSE2 should not be used anywhere, it's always true;
remove -mstackreallign and -mfpmath=sse as useless for x86_64;

Change-Id: Ib7739f6522c375c09de10d22616c48a493841c4c
Signed-off-by: Pavel Chupin <pavel.v.chupin@intel.com>