[MC] Fix llvm_unreachable when a STB_GNU_UNIQUE symbol needs a relocation

STB_GNU_UNIQUE should be treated in a way similar to STB_GLOBAL.
This fixes an "Invalid Binding" failure in an LLVM_ENABLE_ASSERTIONS=on build
for source files like glibc elf/tst-unique1mod1.c .

This bug has been benign so far because (a) Clang does not produce
%gnu_unique_object by itself (b) a non-assertion build likely picks the
STB_GLOBAL code path anyway.