Check m_dyld_up directly in LoadBinariesViaMetadata

In the restructuring I did in https://reviews.llvm.org/D133680 , I
call ObjectFile::LoadBinariesViaMetadata, and the process m_dyld
may be set by a method under there -- in
ProcessMachCore::LoadBinariesViaMetadata I wanted to check to see
if m_dyld_up had been set.  I did this by calling the GetDynamicLoader()
method, but that method will call FindPlugin() if there is no
dynamic loader yet, and the static dynamic loader plugin was being
loaded, preventing the scan for userland binaries in a userland
corefile.

Differential Revision: https://reviews.llvm.org/D137807
rdar://102210820