This brings down the size of the struct from 56 to 32 bytes on 64-bit,
and to 20 bytes on 32-bit. This leads to memory savings:

Before:
$ find . -name 'tcg.o' | xargs size
   text    data     bss     dec     hex filename
  41131   29800      88   71019   1156b ./aarch64-softmmu/tcg/tcg.o
  37969   29416      96   67481   10799 ./x86_64-linux-user/tcg/tcg.o
  39354   28816      96   68266   10aaa ./arm-linux-user/tcg/tcg.o
  40802   29096      88   69986   11162 ./arm-softmmu/tcg/tcg.o
  39417   29672      88   69177   10e39 ./x86_64-softmmu/tcg/tcg.o

After:
$ find . -name 'tcg.o' | xargs size
   text    data     bss     dec     hex filename
  40883   29800      88   70771   11473 ./aarch64-softmmu/tcg/tcg.o
  37473   29416      96   66985   105a9 ./x86_64-linux-user/tcg/tcg.o
  38858   28816      96   67770   108ba ./arm-linux-user/tcg/tcg.o
  40554   29096      88   69738   1106a ./arm-softmmu/tcg/tcg.o
  39169   29672      88   68929   10d41 ./x86_64-softmmu/tcg/tcg.o

Note that using an entire byte for some enums that need less than
that wastes a few bits (noticeable in 32 bits, where we use
20 bytes instead of 16) but avoids extraction code, which overall
is a win--I've tested several variations of the patch, and the appended
is the best performer for OpenSSL's bntest by a very small margin:

Before:
$ taskset -c 0 perf stat -r 15 -- x86_64-linux-user/qemu-x86_64 img/bntest-x86_64 >/dev/null
[...]
 Performance counter stats for 'x86_64-linux-user/qemu-x86_64 img/bntest-x86_64' (15 runs):

      10538.479833 task-clock (msec)  # 0.999 CPUs utilized  ( +-  0.38% )
               772 context-switches   # 0.073 K/sec          ( +-  2.03% )
                 0 cpu-migrations     # 0.000 K/sec          ( +-100.00% )
             2,207 page-faults        # 0.209 K/sec          ( +-  0.08% )
      10.552871687 seconds time elapsed                      ( +-  0.39% )

After:
$ taskset -c 0 perf stat -r 15 -- x86_64-linux-user/qemu-x86_64 img/bntest-x86_64 >/dev/null
 Performance counter stats for 'x86_64-linux-user/qemu-x86_64 img/bntest-x86_64' (15 runs):

      10459.968847 task-clock (msec)  # 0.999 CPUs utilized  ( +-  0.30% )
               739 context-switches   # 0.071 K/sec          ( +-  1.71% )
                 0 cpu-migrations     # 0.000 K/sec          ( +- 68.14% )
             2,204 page-faults        # 0.211 K/sec          ( +-  0.10% )
      10.473900411 seconds time elapsed                      ( +-  0.30% )
Suggested-by: NStefan Weil <sw@weilnetz.de>
Suggested-by: NRichard Henderson <rth@twiddle.net>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NEmilio G. Cota <cota@braap.org>
Signed-off-by: NRichard Henderson <rth@twiddle.net>