arm64/efi: Mark __efistub_stext_offset as an absolute symbol explicitly

After r363059 and r363928 in LLVM, a build using ld.lld as the linker
with CONFIG_RANDOMIZE_BASE enabled fails like so:

ld.lld: error: relocation R_AARCH64_ABS32 cannot be used against symbol
__efistub_stext_offset; recompile with -fPIC

Fangrui and Peter figured out that ld.lld is incorrectly considering
__efistub_stext_offset as a relative symbol because of the order in
which symbols are evaluated. _text is treated as an absolute symbol
and stext is a relative symbol, making __efistub_stext_offset a
relative symbol.

Adding ABSOLUTE will force ld.lld to evalute this expression in the
right context and does not change ld.bfd's behavior. ld.lld will
need to be fixed but the developers do not see a quick or simple fix
without some research (see the linked issue for further explanation).
Add this simple workaround so that ld.lld can continue to link kernels.

Link: https://github.com/ClangBuiltLinux/linux/issues/561
Link: 025a815d75
Link: 249fde8583
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Debugged-by: Fangrui Song <maskray@google.com>
Debugged-by: Peter Smith <peter.smith@linaro.org>
Suggested-by: Fangrui Song <maskray@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
[will: add comment]
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/image.h

@@ -78,7 +78,11 @@
 
 #ifdef CONFIG_EFI
 
-__efistub_stext_offset = stext - _text;
+/*
+ * Use ABSOLUTE() to avoid ld.lld treating this as a relative symbol:
+ * https://github.com/ClangBuiltLinux/linux/issues/561
+ */
+__efistub_stext_offset = ABSOLUTE(stext - _text);
 
 /*
  * The EFI stub has its own symbol namespace prefixed by __efistub_, to