tcg/ppc: Enable tcg backend vector compilation

tcg/ppc: Enable tcg backend vector compilation

Introduce all of the flags required to enable tcg backend vector support,
and a runtime flag to indicate the host supports Altivec instructions.

For now, do not actually set have_isa_altivec to true, because we have not
yet added all of the code to actually generate all of the required insns.
However, we must define these flags in order to disable ifndefs that create
stub versions of the functions added here.

The change to tcg_out_movi works around a buglet in tcg.c wherein if we
do not define tcg_out_dupi_vec we get a declared but not defined Werror,
but if we only declare it we get a defined but not used Werror.  We need
to this change to tcg_out_movi eventually anyway, so it's no biggie.

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>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>