These tests fail when multilib options use -mfpu=xxxx and override the
-mfpu=neon-fp16 used for the test:

  g++.dg/ext/arm-fp16/arm-fp16-ops-5.C
  g++.dg/ext/arm-fp16/arm-fp16-ops-6.C
  gcc.dg/torture/arm-fp16-ops-5.c
  gcc.dg/torture/arm-fp16-ops-6.c
  gcc.target/arm/fp16-compile-vcvt.c

The option -mfpu-neon-fp16 is added via "dg-add-options arm_neon_fp16"
after an earlier "dg-require-effective-target arm_neon_fp16_ok".
This patch modifies check_effective_target_arm_neon_fp16_ok_nocache to
return 0 (causing the test to be skipped) if multilib flags include
-mfpu= with a value other than neon-fp16.

Tested on arm-none-linux-gnueabi for several multilibs; these tests
are compile-only so it didn't matter that I didn't have runtime
support for them.

OK for trunk and 4.6 branch?