tcg/i386: Tidy register constraint definitions

tcg/i386: Tidy register constraint definitions

Create symbolic constants for all low-byte-addressable
and second-byte-addressable registers.  Create a symbol
for the registers that need reserving for softmmu.

There is no functional change for 's', as this letter is
only used for i386.  The BYTEL name is correct for the
action we wish from the constraint.

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