Mark crash_dump.policy as not supported on mips

The recent change to detect missing source files broke reading makefiles
for mips, since this didn't specify a source file.

Bug: 73904572
Test: lunch aosp_mips-eng; m nothing
Test: lunch aosp_arm-eng; m crash_dump.policy
Test: lunch aosp_arm64-eng; m crash_dump.policy
Test: lunch aosp_x86-eng; m crash_dump.policy
Test: lunch aosp_x86_64-eng; m crash_dump.policy
Change-Id: I28864b5af59267f1ab83084128f2c59b04039374