msvcm80: Add __setusermatherr_m stub.

dlls/msvcm80/msvcm80.spec

@@ -300,7 +300,7 @@
 @ stub -arch=win64 ?setw@std@@YA?AU?$_Smanip@_J@1@_J@Z
 @ stub ?signal@@YAP6MXH@ZHH@Z
 @ stub ?signal@@YAP6MXH@ZHP6MXH@Z@Z
-@ stub __setusermatherr_m
+@ cdecl __setusermatherr_m(ptr)
 @ stub towctrans
 @ stub wctrans
 @ stub wctype

dlls/msvcm80/msvcm80_main.c

@@ -49,3 +49,10 @@ void __cdecl CrtImplementationDetails_RegisterModuleUninitializer(void* handler)
 {
     FIXME("%p: stub\n", handler);
 }
+
+/* handler is a "method" with signature int32 (*handler)(_exception*), but I'm
+ * not sure what that means */
+void __cdecl __setusermatherr_m(void *handler)
+{
+    FIXME("%p: stub\n", handler);
+}