arm64: vmlinux.lds: account for spurious empty .igot.plt sections

Now that we started making the linker warn about orphan sections
(input sections that are not explicitly consumed by an output section),
some configurations produce the following warning:

  aarch64-linux-gnu-ld: warning: orphan section `.igot.plt' from
         `arch/arm64/kernel/head.o' being placed in section `.igot.plt'

It could be any file that triggers this - head.o is simply the first
input file in the link - and the resulting .igot.plt section never
actually appears in vmlinux as it turns out to be empty.

So let's add .igot.plt to our collection of input sections to disregard
unless they are empty.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20201028133332.5571-1-ardb@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/vmlinux.lds.S

@@ -278,7 +278,7 @@ SECTIONS
 	 * explicitly check instead of blindly discarding.
 	 */
 	.plt : {
-		*(.plt) *(.plt.*) *(.iplt) *(.igot)
+		*(.plt) *(.plt.*) *(.iplt) *(.igot .igot.plt)
 	}
 	ASSERT(SIZEOF(.plt) == 0, "Unexpected run-time procedure linkages detected!")