IBM Z DFLTCC: fix STFLE usage

IBM Z DFLTCC: fix STFLE usage

QEMU maintainers have found and issue related to incorrect usage of
STFLE instruction [1], which is used to get features supported by the
machine. There are three potential problems with the current usage:

- R0 must contain the number of requested doublewords *minus one*. The
existing code lacks the "minus one" part.
- Older machines may not fill all the doublewords - this is fixed by
calling `memset`.
- STFLE updates R0, but we don't tell the compiler about this - this is
fixed by using a `+` constraint.
- Not really a problem, but it's enough to load 8 bits into R0, so its
type was changed to `uint8_t`. Also, STFLE only writes to `facilities`
variable, therefore memory clobber is unnecessary.

[1] https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg00113.html