stand/efi: Fix for binutils when targeting arm64

When linking with ld.bfd it complain with the following:

/usr/local/bin/aarch64-unknown-freebsd14.0-ld: start.o: relocation
R_AARCH64_ABS32 against `__data_size' can not be used when making a
shared object

Fix this by marking the __data_size with ABSOLUTE. This returns a
non-relocatable value which appears to be the same behaviour of lld.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45257