Rename "llvm-gsym" to "llvm-gsymutil" and fix dependencies.

Summary: This patch renames the "llvm-gsym" tool directory to "llvm-gsymutil". Dependencies are also reduced to the bare minimum for llvm-gsymutil.

Reviewers: aprantl, thakis

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75291