[U-Boot] EFI helloworld build fails with new compilers

[Andre Przywara]

Hi,

after building the just released GCC 7.3.0 compiler I get the below
failure when building U-Boot. This is when building the EFI helloworld
example, which seems to be enabled by default. Manually disabling
CONFIG_CMD_BOOTEFI_HELLO_COMPILE fixes the U-Boot build (but of course
doesn't fix that particular problem).
I see this with origin/master, v2018.01 and v2017.11, but only with GCC
6.4.0 and 7.3.0, but not with 7.1.0. So I guess it's the compiler that
got more picky.

The splat is:
=============
aarch64-linux-ld.bfd: arch/arm/lib/crt0_aarch64_efi.o: relocation
R_AARCH64_ABS16 against `EFI_SUBSYSTEM' can not be used when making a
shared object
scripts/Makefile.lib:399: recipe for target
'lib/efi_loader/helloworld_efi.so' failed
=============

It's beyond me at the moment what causes this or how to fix it, but I
hope you guys can find a solution?

Cheers,
Andre.