Add support for ML64

Add support for ML64

The Microsoft Macro Assembler can be used to assemble and link one
or more assembly-language source files with libtool.

Reported: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=48993

* Makefile.am: Pass ML64 flags to tests.
* configure.ac: Add language support for the Microsoft Macro Assembler.
* doc/libtool.texi: Update documentation to include ML64.
* m4/libtool.m4: Enable macro support for ML64.
* tests/flags.at: Add test for ML64 flags.
* tests/infer-tag.at: Add test for ML64 tag.
* NEWS: Update.