Correct module symbol export handling

EXPORT_SYMS can be set to YES, NO, a list of symbols to export from a
module, or to a filename containing such a list. For the case that it
is set to a symbol list, replace spaces in the list with newlines, so
the created file is in the format expected by kmod_syms.awk.

Reviewed by: imp, jhb
MFC after: 1 month
Sponsored by: Turing Robotic Industries Inc.
Differential Revision: https://reviews.freebsd.org/D14284