[llvm-objdump] --syms: print 'u' for STB_GNU_UNIQUE

GCC when configured with --enable-gnu-unique (default on glibc>=2.11)
emits STB_GNU_UNIQUE for certain objects which are otherwise emitted as
STT_OBJECT, such as an inline function's static local variable or its
guard variable, and a static data member of a template.

Clang does not implement -fgnu-unique.

Implementing it as a binding is strange and the feature itself is
considered by some as a misfeature.

Reviewed By: grimar, jhenderson

Differential Revision: https://reviews.llvm.org/D75797