Bug 1494511 - export aarch64 windows xptcstubs from their source file; r=dmajor

DONTBUILD because NPTOB.

xpcom/reflect/xptcall/md/win32/xptcstubs_asm_aarch64.asm

@@ -39,6 +39,7 @@ NFPREGS EQU 8
 
 	MACRO
 	STUBENTRY $functionname, $paramcount
+	EXPORT |$functionname|
 |$functionname| PROC
 	mov	w17, $paramcount
 	b	SharedStub