s390/purgatory: Fix endless interrupt loop

New compilers use the floating-point registers as spill registers when
there is high register pressure. In the purgatory however, the afp control
bit is not set. This leads to an exception whenever a floating-point
instruction is used, which again causes an interrupt loop.

Forbid the compiler to use floating-point instructions by adding
-msoft-float to KBUILD_CFLAGS.
Signed-off-by: Philipp Rudo <prudo@linux.ibm.com>
Fixes: 840798a1 (s390/kexec_file: Add purgatory)
Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/purgatory/Makefile

@@ -21,7 +21,7 @@ LDFLAGS_purgatory.ro += -z nodefaultlib
 KBUILD_CFLAGS := -fno-strict-aliasing -Wall -Wstrict-prototypes
 KBUILD_CFLAGS += -Wno-pointer-sign -Wno-sign-compare
 KBUILD_CFLAGS += -fno-zero-initialized-in-bss -fno-builtin -ffreestanding
-KBUILD_CFLAGS += -c -MD -Os -m64
+KBUILD_CFLAGS += -c -MD -Os -m64 -msoft-float
 KBUILD_CFLAGS += $(call cc-option,-fno-PIE)
 
 $(obj)/purgatory.ro: $(PURGATORY_OBJS) FORCE