x86: conditionally hide object-format-specific functions

x86: conditionally hide object-format-specific functions

ELF-only functions don't need to be built when dealing with a non-ELF
target. md_section_align() also doesn't need to be a function when
dealing with non-AOUT targets. Similarly tc_fix_adjustable() can be a
simple macro when building non-ELF targets.

Furthermore x86_elf_abi is already used in ELF-only code sections, with
one exception. By adjusting that, the otherwise bogusly named variable
can also be confined to just ELF builds.