[LTO] Parse deplibs sections for possible libcalls.
LTO codegen can add symbol references resolved by object files with
.deplibs sections. These object files will be added to the link by
symbol resolution, but parseFile() will never be called on the object
file, so the .deplibs will never be resolved. This can lead to undefined
This change scans the deplibs sections of such object files before LTO
occurs and summarily adds these libraries to the link. Shared libraries
are added as if --as-needed were specified, since they may not end up
actually being required. Similarly, shared libraries will only be
extracted if something actually ends up referencing the newly added
Since the problem was first observed on AArch64-specific libcalls, this
also adds an ELF lld target interface for obtaining target-specific
libcalls (those corresponding to setLibcallName() calls in target
lowering). These are filled in for AArch64.