[testsuite] XFAIL gcc.dg/tree-ssa/ssa-dom-cse-2.c on non-NEON arm targets

[testsuite] XFAIL gcc.dg/tree-ssa/ssa-dom-cse-2.c on non-NEON arm targets

This test fails to optimise away the PLUS reduction in the loop on arm targets when vectorisation
is not enabled due to absence of SIMD instructions.
From reading the logs and the PR I gather that the presence or absence of SIMD affects the passing of this test
on other targets as well, as evidenced by the long list of xfail targets.
This list looks quite unwieldy to me, but here is a patch adding non-NEON arm to that list.

    * gcc.dg/tree-ssa/ssa-dom-cse-2.c: XFAIL on !arm_neon arm targets.

From-SVN: r257175