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.

Signed-off-by: Richard Henderson <rth@twiddle.net>
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.

Signed-off-by: Richard Henderson <rth@twiddle.net>