[ELF][Morello] Add basic support for LTO

When parsing bitcode files, ld.lld would previously hit an assertion due to
a mismatched DataLayout. This patch adds minimal support for Morello LTO
by inferring the target ABI from the morello-c64-plt flag. This flag
is set automatically based on the ELF e_flags, so as long as the first
input file is a purecap file, it should also work automatically when using
a bare ld.lld invocation instead calling ld.lld via clang.

This is lightly tested, but I have been able to build various projects
with -flto=thin.