x86/boot: Fix a comment's incorrect file reference

x86/boot: Fix a comment's incorrect file reference

Fix the comment for 'struct real_mode_header' to reference the correct
assembly file, realmode/rm/header.S.  The comment has always incorrectly
referenced realmode.S, which doesn't exist, as defining the associated
asm blob.

Specify the file's path relative to arch/x86 to avoid confusion with
boot/header.S.  Update the comment for 'struct trampoline_header' to
also include the relative path to keep things consistent, and tweak the
dual 64/32 reference so that it doesn't appear to be an extension of the
relative path, i.e. avoid "realmode/rm/trampoline_32/64.S".

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20191126195911.3429-1-sean.j.christopherson@intel.com