Disable integrated-as for Mips

Bug: http://b/29771478

Hand-coded assembly in various projects will fail to compile after Clang
switches to integrated assembler for Mips.  Disable integrated-as for
Mips.

Test: Verified Mips builds on AOSP both with the old Clang prebuilts and
the new prebuilts (https://android-review.googlesource.com/#/c/243871/)

Change-Id: Idf441cb9c6c2afc8997d5ee56420885930087918