ELF: Report original emulation name in "unknown emulation" error

A trailing _fbsd is stripped from emulation names, but if the result was
still not a valid emulation the error was somewhat confusing.

For example,

% ld.lld -m elf_amd64_fbsd
unknown emulation: elf_amd64

Use the original emulation name in error messages.

Differential Revision:	https://reviews.llvm.org/D24357

llvm-svn: 280983