tcg-arm: Simplify logic in detecting the ARM ISA in use GCC 4.8 defines a handy __ARM_ARCH symbol that we can use, which will make us nicely forward compatible with ARMv8 AArch32. Reviewed-by: Peter Maydell <peter.maydell@linaro.org> Signed-off-by: Richard Henderson <rth@twiddle.net>