tcg-sparc: Use TCG_TARGET_REG_BITS in conditional compilation.

The test TCG_TARGET_REG_BITS==64 is exactly the feature that we
are checking for, whereas something involving __sparc_v9__ or
__sparc_v8plus__ should be reserved for something ISA related,
as with SMULX.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
