[llvm-objdump] support '--syms' as an alias of -t

This adds support for '--syms' as an alias of '-t' for llvm-objdump,
fixing PR39406 (https://bugs.llvm.org/show_bug.cgi?id=39406).

Patch by Higuoxing (Xing).

Differential Revision: https://reviews.llvm.org/D53803

llvm-svn: 345697