[PECOFF] Acquire mutex before accessing shared objects.

addResolvableSymbols() queues input files, and readAllSymbols() reads
from them. In practice it's currently safe because they are called from
a single thread. But it's not guaranteed.

Also, acquiring the same mutex is needed not to see inconsistent memory
contents that is allowed in the C++ memory model.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@209254 91177308-0d34-0410-b5e6-96231b3b80d8