x86/um/vdso: Drop implicit common-page-size linker flag

GNU linker's -z common-page-size's default value is based on the target
architecture. arch/x86/um/vdso/Makefile sets it to the architecture
default, which is implicit and redundant. Drop it so that one more LLVM
build issue gets addressed.

Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Richard Weinberger <richard@nod.at>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-um@lists.infradead.org
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20181206191231.192355-1-ndesaulniers@google.com