At least libcutils and boringssl need to set source files that
apply to x86 or x86_64, but only on the host or on the device.
Add new properties that can contain arch variant properties:
target.android_arm
target.android_arm64
target.android_mips
target.android_mips64
target.android_x86
target.android_x86_64
target.linux_x86
target.linux_x86_64
target.darwin_x86
target.darwin_x86_64
Change-Id: I5a7076653a7367a63daa7f7e34a6a28f5cbdfbe7
b05bff2f26