stand/efi: Fix for binutils when targeting arm64

[andrew]

Summary

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.

Sponsored by: Arm Ltd

[imp]

Even though we relocate this binary, this value is invariant. And less than 32 bits, though that shouldn't matter here