Set generic_x86/BoardConfig.mk TARGET_ARCH_VARIANT to x86

Change TARGET_ARCH_VARIANT from x86-atom to x86, for the reason
that atom-specific instruciton like "movbe" isn't "generic" to
x86 Android devices and x86 host running emulator-x86, which may
crash non-atom host in VT because "movbe" isn't supported natively.

Also revert previous fix in TARGET_linux-x86.mk which conservatively
disable "movbe" across the board

Change-Id: Ief93a3585566ffae558fcdc29741e6213a048d7d