aarch64: Skip some SME register save tests on BE

aarch64: Skip some SME register save tests on BE

Big-endian targets need to save Z8-Z15 in the same order as
the registers would appear for D8-D15, because the layout is
mandated by the EH ABI.  BE targets therefore use ST1D instead
of the normal STR for those registers (but not for others).

That difference is already tested elsewhere and isn't important
for the SME tests.  This patch therefore restricts the affected
tests to LE.

gcc/testsuite/
	* gcc.target/aarch64/sme/call_sm_switch_5.c: Restrict tests that
	contain Z8-Z23 saves to little-endian.
	* gcc.target/aarch64/sme/call_sm_switch_8.c: Likewise.
	* gcc.target/aarch64/sme/locally_streaming_1.c: Likewise.