This patch makes the VIXL_INCLUDE_SIMULATOR and
VIXL_GENERATE_SIMULATOR_CODE header guards specific to either AArch64 or
AArch32. Even though the simulator only support AArch64. The build
system was updated accordingly, the "simulator" variable now takes
"aarch64" or "none" as possible values instead of "on" and "off".
This fixes issues we have when we want to build VIXL natively on
AArch64 without a simulator, but still include the AArch32
macro-assembler. The later would check for VIXL_GENERATE_SIMULATOR_CODE
and then generate calls to native code, which breaks.
Change-Id: I2850782558d4cc37f37c1644f0efbd70a3123057