Fix the mips64 world build after r364284.

Linking the full version of clang 11 results in errors similar to:

lld: error: /usr/src/contrib/llvm-project/clang/lib/StaticAnalyzer/Frontend/AnalysisConsumer.cpp:736:(.text._ZN5clang4ento22CreateAnalysisConsumerERNS_16CompilerInstanceE+0xE0): relocation R_MIPS_CALL16 out of range: 48920 is not in [-32768, 32767]; references operator new(unsigned long)

Add -mxgot to the compilation flags for llvm libraries to work around
this error. This may be too big of a hammer, but it can always be
refined later.

MFC after: 6 weeks