Configuration: add unsupported masm target.

Reviewed-by: NRichard Levitte <levitte@openssl.org>

Configurations/50-masm.conf

+# We can't make commitment to supporting Microsoft assembler,
+# because it would mean supporting all masm versions. This in
+# in turn is because masm is not really an interchangeable option,
+# while users tend to have reasons to stick with specific Visual
+# Studio versions. It's usually lesser hassle to make it work
+# with latest assembler, but tweaking for older versions had
+# proven to be daunting task. This is experimental target, for
+# production builds stick with [up-to-date version of] nasm.
+
+%targets = (
+    "VC-WIN64A-masm" => {
+        inherit_from    => [ "VC-WIN64A" ],
+        as              => "ml64",
+        asflags         => "/c /Cp /Cx /Zi",
+        asoutflags      => "/Fo",
+    },
+);