cpu/x86/64bit/Makefile: use all_x86 make target

Use the newly introduced 'all_x86' make target to add the mode_switch.S
compilation unit to all stages that run on the x86 cores, but not to
verstage on PSP.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I8950375d31557d9a38169869c1d250417261c31c
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74197
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>

src/cpu/x86/64bit/Makefile.inc

@@ -1,8 +1,4 @@
-bootblock-y += mode_switch.S
-verstage_x86-y += mode_switch.S
-romstage-y += mode_switch.S
-postcar-y += mode_switch.S
-ramstage-y += mode_switch.S
+all_x86-y += mode_switch.S
 
 # Add --defsym=_start=0 to suppress a linker warning.
 $(objcbfs)/pt: $(dir)/pt.S $(obj)/config.h