x86/HyperV: correct hv_hcall_page for xen.efi build

Along the lines of what the not reverted part of 3c4b2eef49 ("x86:
refine link time stub area related assertion") did, we need to transform
the absolute HV_HCALL_PAGE into the image base relative hv_hcall_page
(or else there'd be no need for two distinct symbols). Otherwise
mkreloc, as used for generating the base relocations of xen.efi, will
spit out warnings like "Difference at .text:0009b74f is 0xc0000000
(expected 0x40000000)". As long as the offending relocations are PC
relative ones, the generated binary is correct afaict, but if there ever
was the absolute address stored, xen.efi would miss a fixup for it.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wl@xen.org>

xen/arch/x86/xen.lds.S

@@ -327,7 +327,7 @@ SECTIONS
 #endif
 
 #ifdef CONFIG_HYPERV_GUEST
-  hv_hcall_page = ABSOLUTE(HV_HCALL_PAGE);
+  hv_hcall_page = ABSOLUTE(HV_HCALL_PAGE - XEN_VIRT_START + __XEN_VIRT_START);
 #endif
 
   /* Sections to be discarded */