aarch64, testsuite: Tweak sve/pcs/args_9.c to allow stps

aarch64, testsuite: Tweak sve/pcs/args_9.c to allow stps

With the new ldp/stp pass enabled, there is a change in the codegen for
this test as follows:

        add     x8, sp, 16
        ptrue   p3.h, mul3
        str     p3, [x8]
-       str     x8, [sp, 8]
-       str     x9, [sp]
+       stp     x9, x8, [sp]
        ptrue   p3.d, vl8
        ptrue   p2.s, vl7
        ptrue   p1.h, vl6

i.e. we now form an stp that we were missing previously. This patch
adjusts the scan-assembler such that it should pass whether or not
we form the stp.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/sve/pcs/args_9.c: Adjust scan-assemblers to
	allow for stp.