[LLD] [COFF] Add support for a new, mingw specific embedded directive -exclude-symbols:

This is an entirely new embedded directive - extending the GNU ld
command line option --exclude-symbols to be usable in embedded
directives too.

(GNU ld.bfd also got support for the same new directive, currently in
the latest git version, after the 2.39 branch.)

This works as an inverse to the regular embedded dllexport directives,
for cases when autoexport of all eligible symbols is performed.

Differential Revision: https://reviews.llvm.org/D130120