efi: restrict visibility of EFIABI_ATTR-declared functions

In-tree gcc (4.2) doesn't understand attribute((ms_abi))
(EFIABI_ATTR). Avoid declaring functions with that attribute when the
compiler is detected to be gcc < 4.4.

Reviewed by: kib, imp (previous version)
Approved by: markj (mentor)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D11636