tcg: Always define all of the TCGOpcode enum members.

By always defining these symbols, we can eliminate a lot of ifdefs.

To allow this to be checked reliably, the semantics of the
TCG_TARGET_HAS_* macros must be changed from def/undef to true/false.
This allows even more ifdefs to be removed, converting them into
C if statements.

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