ARM: make gd a function for LTO

On ARM the gd pointer is stored in registers r9 / x18. For this the
-ffixed-r9 / -ffixed-x18 flag when compiling, but using global register
variables causes errors when building with LTO, and these errors are
very difficult to overcome.

Richard Biener says [1]:
  Note that global register vars shouldn't be used with LTO and if they
  are restricted to just a few compilation units the recommended fix is
  to build those CUs without -flto.

We cannot do this for U-Boot since all CUs use -ffixed-reg flag.

It seems that with LTO we could in fact store the gd pointer differently
and gain performance or size benefit by allowing the compiler to use
r9 / x18. But this would need more work.

So for now, when building with LTO, go the clang way, and instead of
declaring gd a global register variable we make it a function via macro.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68384



Signed-off-by: Marek Behún <marek.behun@nic.cz>