arm64: relocatable: Tweak linker flags to yield a smaller Image with LLD

Currently, there is a regression of 689 KiB in Image.gz's size when the
kernel is linked with LLD. This is reduced to 213 KiB when we use -pie
rather than -shared when invoking the linker.

Unfortunately, ld.bfd dislikes this change and regresses in size by 163
KiB with -pie as compared to using -shared. To address this problem, we
add checks so that -pie is used with LLD and -shared is used with
ld.bfd. That way, both linkers are able to perform their best.

List of Image.gz sizes:
  ld.bfd -shared: 10,066,988 bytes
  ld.bfd -pie:    10,230,316 bytes
  LLD -shared:    10,796,872 bytes
  LLD -pie:       10,280,168 bytes

Test: kernel compiles and boots with both ld.bfd and LLD
Signed-off-by: Danny Lin <danny@kdrag0n.dev>
[@0ctobot: Simplify checks so that ld.gold remains unaffected]
Signed-off-by: Adam W. Willis <return.of.octobot@gmail.com>

arch/arm64/Makefile

@@ -18,8 +18,15 @@ ifeq ($(CONFIG_RELOCATABLE), y)
 # Pass --no-apply-dynamic-relocs to restore pre-binutils-2.27 behaviour
 # for relative relocs, since this leads to better Image compression
 # with the relocation offsets always being zero.
-LDFLAGS_vmlinux		+= -shared -Bsymbolic -z notext -z norelro \
+LDFLAGS_vmlinux		+= -Bsymbolic -z notext -z norelro \
 			$(call ld-option, --no-apply-dynamic-relocs)
+
+# Pass -pie for lld to minimize image size
+ifeq ($(ld-name),lld)
+LDFLAGS_vmlinux		+= -pie
+else
+LDFLAGS_vmlinux		+= -shared
+endif
 endif
 
 ifeq ($(CONFIG_ARM64_ERRATUM_843419),y)