testsuite: Fix expand-return CMSE test for Armv8.1-M [PR115253]

For Armv8.1-M, the clearing of the registers is handled differently than
for Armv8-M, so update the test case accordingly.

gcc/testsuite/ChangeLog:

	PR target/115253
	* gcc.target/arm/cmse/extend-return.c: Update test case
	condition for Armv8.1-M.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>
Co-authored-by: Yvan ROUX <yvan.roux@foss.st.com>