lib/stack: Restrengthen base_address

commit a1f2b0e1 ("treewide: lib/stack: Make base_address arch
specific") made base_address() a weak function in order to allow
architectures to override it. Linking for EFI doesn't seem to figure
out the right one to use though [anymore?]. It must have worked at
one point because the commit calls outs EFI as the motivation.
Anyway, just drop the weakness in favor of another HAVE_ define.

Signed-off-by: Andrew Jones <andrew.jones@linux.dev>