tcg-i386: Introduce limited deposit support

x86 cannot provide an optimized generic deposit implementation. But at
least for a few special cases, namely for writing bits 0..7, 8..15, and
0..15, versions using only a single instruction are feasible.
Introducing such limited support improves emulating 16-bit x86 code on
x86, but also rarer cases where 32-bit or 64-bit code accesses bytes or
words.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
