Add -z separate-code to default ARM64 device flags

Removing XOM had the side effect of removing "-z separate-code", which
was needed to override a new default after a recent toolchain update.
This led to some performance regressions in some tests. For now, add
this flag to the global arm64 device flags to return to the previous
behavior.

Bug: 150506341
Test: m -j
Change-Id: I54787954141d133e245dfd259a37bf4c3c8e7caa

cc/config/arm64_device.go

@@ -39,6 +39,7 @@ var (
 	arm64Ldflags = []string{
 		"-Wl,-m,aarch64_elf64_le_vec",
 		"-Wl,--hash-style=gnu",
+		"-Wl,-z,separate-code",
 		"-fuse-ld=gold",
 		"-Wl,--icf=safe",
 	}