x86/ELF: don't store function pointer in elf_core_save_regs()

x86/ELF: don't store function pointer in elf_core_save_regs()

This keeps at least gcc 10 from generating a separate function instance
in common/kexec.o alongside the inlining of the function in its sole
caller. I also think putting the address of the actual code storing the
registers is a better indication to consumers than that of an otherwise
unreferenced function.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>