testsuite: Restrict mpc860_no_lwsync.c to Power ilp32 targets [PR125448]

The recently added mpc860_no_lwsync.c test case fails on 64-bit PowerPC
targets (such as powerpc64le-linux-gnu) because the default -m64 option
conflicts with the 32-bit legacy processor specified by -mcpu=860,
resulting in the error: "cc1: error: '-m64' requires a PowerPC64 cpu".

Fix this by restricting the test case execution to 32-bit PowerPC
targets using the 'ilp32' target requirement.

2026-05-27  Kishan Parmar  <kishan@linux.ibm.com>
	    Surya Kumari Jangala  <jskumari@linux.ibm.com>

gcc/testsuite/ChangeLog:
	PR target/125448
	* gcc.target/powerpc/mpc860_no_lwsync.c: Restrict to ilp32 targets.