Implement Loongson 2F BTB bug workaround in clang

This adapts the -mfix-loongson2f-btb workaround from as(1) to
LLVM/clang. Because LLVM emits machine code directly without using
assembly as interme

Implement Loongson 2F BTB bug workaround in clang

This adapts the -mfix-loongson2f-btb workaround from as(1) to
LLVM/clang. Because LLVM emits machine code directly without using
assembly as intermediary form, the workaround has to be applied
in the compiler backend in addition to the integrated assembler.

Discussed with miod@
OK mortimer@

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