[MinGW] Correct handling different forms of a few options

Support the equals form of the long --entry=<symbol> option,
add a test for the -e<symbol> form.

Add tests for single dash forms of -exclude-all-symbols and
-export-all-symbols.

Support single-dash forms of -out-implib and -output-def, support
the equals form of --output-def=<file>. (We previously had a test
to explicitly disallow -out-implib, but it turns out that GNU ld
actually does support it just fine, despite also matching the
-o<file> option.)

Disallow the double-dashed --u form, add a test for -u<symbol>.

Differential Revision: https://reviews.llvm.org/D66066

llvm-svn: 368816