testsuite/102690: Only check warning for lp64 in Warray-bounds-16.C

That warning won't happen on ilp32 targets, seems like Andrew Pinski
already mention that[1] before.

Verified on riscv32-unknown-elf and riscv64-unknown-elf.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92879#c1

gcc/testsuite/ChangeLog:

	PR testsuite/102690
	* g++.dg/warn/Warray-bounds-16.C: XFAIL only on lp64 for the
	warning.