[LLD] Allow configuring default ld.lld backend

The motivation for this is ld.lld --help targeting MinGW which
currently prints help for the ELF backend unless -m i386pe{,p} is
added. This confuses build systems that grep through linker help to
find supported flags.

This matches LD from Binutils which always prints help for MinGW
when configured to target it.

After this change, the backend can still be overridden to any
supported ELF/MinGW target by using correct -m <arch>.

Differential Revision: https://reviews.llvm.org/D87418