tcg/ppc: Replace HAVE_ISEL macro with a variable

Previously we've been hard-coding knowledge that Power7 has ISEL, but
it was an optional instruction before that.  Use the AT_HWCAP2 bit,
when present, to properly determine support.

Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>