[build] Refuse to attempt building with the GNU gold linker

[build] Refuse to attempt building with the GNU gold linker

GNU gold (part of newer binutils builds) does not appear to be
designed to support generic linker functionality, since its source
code contains several Linux-specific hard-coded assumptions about the
layout of ELF binaries.  Attempting to build iPXE using GNU gold will
generally cause some kind of "linker internal error".

Provide an explicit error message suggesting the use of GNU ld
instead.

Reported-by: Chris Hills <chaz@chaz6.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>