[AMDGPU] Adding mutex to guard lld::elf::link interface use

check-mlir target run tests simultaneously with multiple threads. This caused multiple threads to invoke the `lld::elf::link()` interface at the same time. Since the interface does not have a thread-safe implementation, add a metex to prevent multi-threaded access.

I discovered this by looking the the failure stack trace. lld/ELF/symbolTable.cpp, SymbolTable::insert() hit into an assert with related to Epoch Trackers. The root cause is to due to there is no protection around the symMap (update) which is implemented in non-thread safe data structure: denseMap.

Differential Revision: https://reviews.llvm.org/D88038