debug/elf: add missing EM_AARCH64 to machineStrings

EM_AARCH64 is defined as a constant, but the corresponding entry in
machineStrings is missing. Add it.

Change-Id: I6506404386efe608877095e635a290bbc0686215
Reviewed-on: https://go-review.googlesource.com/106035
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/debug/elf/elf.go

@@ -289,6 +289,7 @@ var machineStrings = []intName{
 	{60, "EM_ST100"},
 	{61, "EM_TINYJ"},
 	{62, "EM_X86_64"},
+	{183, "EM_AARCH64"},
 
 	/* Non-standard or deprecated. */
 	{6, "EM_486"},