x86/efi: move the logic to detect PE build support

So that it can be used by other components apart from the efi specific
code. By moving the detection code creating a dummy efi/disabled file
can be avoided.

This is required so that the conditional used to define the efi symbol
in the linker script can be removed and instead the definition of the
efi symbol can be guarded using the preprocessor.

The motivation behind this change is to be able to build Xen using lld
(the LLVM linker), that at least on version 6.0.0 doesn't work
properly with a DEFINED being used in a conditional expression:

ld    -melf_x86_64_fbsd  -T xen.lds -N prelink.o --build-id=sha1 \
    /root/src/xen/xen/common/symbols-dummy.o -o /root/src/xen/xen/.xen-syms.0
ld: error: xen.lds:233: symbol not found: efi

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Daniel Kiper <daniel.kiper@oracle.com>

.gitignore

@@ -302,7 +302,6 @@ xen/arch/x86/boot/*.bin
 xen/arch/x86/boot/*.lnk
 xen/arch/x86/efi.lds
 xen/arch/x86/efi/check.efi
-xen/arch/x86/efi/disabled
 xen/arch/x86/efi/mkreloc
 xen/arch/*/efi/boot.c
 xen/arch/*/efi/compat.c