testsuite: arm: Use effective target for pr57735.C test

Update test cases to use -mcpu=unset/-march=unset feature introduced in
r15-3606-g7d6c6a0d15c.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/pr57735.C: Use effective-target arm_cpu_xscale_arm.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>