x86/purgatory: add missing FORCE to Makefile target

- Build the kernel without the fix
- Add some flag to the purgatories KBUILD_CFLAGS,I used
  -fno-asynchronous-unwind-tables
- Re-build the kernel

When you look at makes output you see that sha256.o is not re-build in the
last step.  Also readelf -S still shows the .eh_frame section for
sha256.o.

With the fix sha256.o is rebuilt in the last step.

Without FORCE make does not detect changes only made to the command line
options.  So object files might not be re-built even when they should be.
Fix this by adding FORCE where it is missing.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: df6f280 ("kernel/kexec_file.c: move purgatories sha256 to common code")
Signed-off-by: Philipp Rudo <[email protected]>
Acked-by: Dave Young <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: <[email protected]>	[4.17+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Philipp Rudo

arch/x86/purgatory/Makefile

@@ -6,7 +6,7 @@ purgatory-y := purgatory.o stack.o setup-x86_$(BITS).o sha256.o entry64.o string
 targets += $(purgatory-y)
 PURGATORY_OBJS = $(addprefix $(obj)/,$(purgatory-y))
 
-$(obj)/sha256.o: $(srctree)/lib/sha256.c
+$(obj)/sha256.o: $(srctree)/lib/sha256.c FORCE
 	$(call if_changed_rule,cc_o_c)
 
 LDFLAGS_purgatory.ro := -e purgatory_start -r --no-undefined -nostdlib -z nodefaultlib