target/ppc: switch FPR, VMX and VSX helpers to access data directly from cpu_env

target/ppc: switch FPR, VMX and VSX helpers to access data directly from cpu_env

Instead of accessing the FPR, VMX and VSX registers through static arrays of
TCGv_i64 globals, remove them and change the helpers to load/store data directly
within cpu_env.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>