[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.