tcg-arm: Use AT_PLATFORM to detect the host ISA

With this we can generate armv7 insns even when the OS compiles for a
lower common denominator.  The macros are arranged so that when we do
compile for a given ISA, all of the runtime checks for that ISA are
optimized away.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
