Refine fusefs test workaround for mips+llvm

This re-enables building the googletest suite by default on mips and instead
specifically doesn't build fusefs tests for mips+clang builds. clang will
easily spent >= 1.5 hours compiling a single file due to a bug in
optimization (see LLVM PR 43263), so turn these off for now while that's
hashed out.

GCC builds are unaffected and build the fusefs tests as-is. Clang builds
only happen by early adopters attempting to hash out the remaining issues.

The comment has been updated to reflect its new position and use less strong
wording about imposing on people.

Discussed with: ngie, asomers
Reviewed by: ngie