cgen: add callconv support for fns from ptr (#14151)

Author: fleur

vlib/v/gen/c/assign.v

@@ -314,11 +314,32 @@ fn (mut g Gen) gen_assign_stmt(node_ ast.AssignStmt) {
 				}
 				func := right_sym.info as ast.FnType
 				ret_styp := g.typ(func.func.return_type)
-				g.write('$ret_styp (*${g.get_ternary_name(ident.name)}) (')
+
+				mut call_conv := ''
+				mut msvc_call_conv := ''
+				for attr in func.func.attrs {
+					match attr.name {
+						'callconv' {
+							if g.is_cc_msvc {
+								msvc_call_conv = '__$attr.arg '
+							} else {
+								call_conv = '$attr.arg'
+							}
+						}
+						else {}
+					}
+				}
+				call_conv_attribute_suffix := if call_conv.len != 0 {
+					'__attribute__(($call_conv))'
+				} else {
+					''
+				}
+
+				g.write('$ret_styp ($msvc_call_conv*${g.get_ternary_name(ident.name)}) (')
 				def_pos := g.definitions.len
 				g.fn_decl_params(func.func.params, voidptr(0), false)
 				g.definitions.go_back(g.definitions.len - def_pos)
-				g.write(')')
+				g.write(')$call_conv_attribute_suffix')
 			} else {
 				if is_decl {
 					if is_inside_ternary {