swift234

build: only use gold on ELF targets

gold only supports ELF.  This adds two new helper functions
(is_windows_based_sdk and is_elfish_sdk) to ensure that we dont try to use gold
on non-ELF targets.  This comes up when trying to setup cross-compilation for
the standard library for Windows.

The ELF check is implemented as the negation of Darwin (which uses MachO) and
Windows (which uses COFF).  The reason for this is that there are additional
targets which also use ELF.  Rather than enumerating the larger set, enumerate
the smaller set (windows) and use the negation.