clang version 6.0.0 (trunk 310694) is able to compile MSA code.
Previous versions had an issue with _msa_fill_w(v32)
In this CL the macro DISABLE_CLANG_MSA is not set, allowing clang
to build the full MSA source.

TBR=kjellander@chromium.org
BUG=libyuv:715
TEST=gn gen out/Release "--args=is_debug=false target_os=\"android\" target_cpu=\"mips64el\" mips_arch_variant=\"r6\" mips_use_msa=true is_component_build=true is_clang=true"
ninja -v -C out/Release libyuv_unittest

Change-Id: I47401e3b1a3e4c57d9626ec2d3cd131c3ccf613c
Reviewed-on: https://chromium-review.googlesource.com/656501Reviewed-by: Frank Barchard <fbarchard@google.com>