gas: scfi: fix failing test on Solaris2

It has been observed that the run of scfi-unsupported-1 test with --x32
arg on a Solaris2 x86_64 system fails:

Executing on host: sh -c {../as-new  --x32 --scfi=experimental \
          <...>/scfi-unsupported-1.s 2>&1}  /dev/null dump.out (timeout = 300)
Assembler messages:
Fatal error: no compiled in support for 32bit x86_64
regexp_diff match failure
regexp "^Fatal error: SCFI is not supported for this ABI$"
line   "Fatal error: no compiled in support for 32bit x86_64"
FAIL: x86_64 scfi-unsupported-1

Fix the above by adding a check for --x32 support before running the
test.  While at it, also include a similar check for --32 support.

gas/testsuite/
	* gas/scfi/x86_64/scfi-x86-64.exp: Add gas_x32_check and
	gas_32_check.  Conditionalize the execution of affected
	testcases.