[s390x] Add missing condition code clobbers to inline assembly

For x86, GCC will assume that any inline assembly instruction clobbers
the condition code and so it is not necessary to explicitly include
"cc" within the clobber list.

For s390x, GCC will assume that the condition code is preserved unless
explicitly clobbered.  It is extremely rare for GCC to emit code that
actually relies upon this behaviour, but it has been observed to
happen within a series of memcpy() calls with condition-dependent
source addresses.

Fix by adding "cc" to the clobber list on all inline assembly blocks
where the condition code is not guaranteed to be preserved.

It would be possible to use "=@cc" as an output constraint to obtain
the carry result from bigint_add() and bigint_subtract(), but the
instructions generated by GCC to extract the condition code end up
being larger than the single "alcr"/"slbr" that we currently use to
obtain the carry result.

Signed-off-by: Michael Brown <mcb30@ipxe.org>