x86: Fix call64's section flags

x86: Fix call64's section flags

When a section is not flagged with SHF_ALLOC, LLD's --gc-sections
algorithm fails to visit the sections that it references. As a result of
this, LLD was dropping the call64.o(.data) section, which is itself only
referenced by .text_call64.

This appears to be a bug in LLD, but the .section directive for
.text_call64 should really have the correct flags either way.

Add `"ax"` to mark the section as ALLOC ("supposed to be loaded") and
CODE ("supposed to be executed").

Fixes: 7dc82591d68e2a ("x86: Move call64 into its own section")
Signed-off-by: Sam Edwards <CFSworks@gmail.com>