aarch64: Prevent streaming-compatible code from assembler rejection [PR121028]

aarch64: Prevent streaming-compatible code from assembler rejection [PR121028]

Streaming-compatible functions can be compiled without SME enabled, but need
to use "SMSTART SM" and "SMSTOP SM" to temporarily switch into the streaming
state of a callee.  These switches are conditional on the current mode being
opposite to the target mode, so no SME instructions are executed if SME is not
available.

However, in GAS, "SMSTART SM" and "SMSTOP SM" always require +sme.  A call
from a streaming-compatible function, compiled without SME enabled, to a non
-streaming function will be rejected as:

Error: selected processor does not support `smstop sm'..

To work around this, we make use of the .inst directive to insert the literal
encodings of "SMSTART SM" and "SMSTOP SM".

gcc/ChangeLog:
	PR target/121028
	* config/aarch64/aarch64-sme.md (aarch64_smstart_sm): Use the .inst
	directive if !TARGET_SME.
	(aarch64_smstop_sm): Likewise.

gcc/testsuite/ChangeLog:
	PR target/121028
	* gcc.target/aarch64/sme/call_sm_switch_1.c: Tell check-function
	-bodies not to ignore .inst directives, and replace the test for
	"smstart sm" with one for it's encoding.
	* gcc.target/aarch64/sme/call_sm_switch_11.c: Likewise.
	* gcc.target/aarch64/sme/pr121028.c: New test.

(cherry picked from commit d52e9ef98bb30872482a46e7a2ec6a20c3ca4a4c)