Use SSE2 enhanced memset for capable x86 processors

Originally, if TARGET_ARCH_VARIANT is x86-atom, the SSE2 enhanced
memset is used. This patch extends this to all x86 processors which
support SSE2 (i.e. ARCH_X86_HAVE_SSE2 is true).

Indentation added to the ifeq cases to make this easier to read.

Change-Id: I05f49e237a95359d3f2e3216b037e3fc1a0fbcb0
Signed-off-by: Daniel Leung <daniel.leung@intel.com>
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>