tcg/i386: Split out target constraints to tcg-target-con-str.h

tcg/i386: Split out target constraints to tcg-target-con-str.h

This eliminates the target-specific function target_parse_constraint
and folds it into the single caller, process_op_defs.  Since this is
done directly into the switch statement, duplicates are compilation
errors rather than silently ignored at runtime.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>