s390/boot: do not emit debug info for assembly with llvm's IAS

Commit ee6d777d ("s390/decompressor: support extra debug flags")
added extra debug flags, in particular debug info is created,
depending on config options.

With llvm's IAS this causes this compile warning:

arch/s390/boot/head.S:38:1: warning: DWARF2 only supports one section per compilation unit
.section ".head.text","ax"
^

This is a known problem and was addressed with commit b8a90923
("Kbuild: do not emit debug info for assembly with LLVM_IAS=1").
Just do the same for s390 to get rid of this warning.
Tested-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20220511120532.2228616-8-hca@linux.ibm.comSigned-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/Makefile

@@ -20,7 +20,9 @@ LDFLAGS_vmlinux	:= -pie
 endif
 aflags_dwarf	:= -Wa,-gdwarf-2
 KBUILD_AFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -D__ASSEMBLY__
+ifndef CONFIG_AS_IS_LLVM
 KBUILD_AFLAGS_DECOMPRESSOR += $(if $(CONFIG_DEBUG_INFO),$(aflags_dwarf))
+endif
 KBUILD_CFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -O2 -mpacked-stack
 KBUILD_CFLAGS_DECOMPRESSOR += -DDISABLE_BRANCH_PROFILING -D__NO_FORTIFY
 KBUILD_CFLAGS_DECOMPRESSOR += -fno-delete-null-pointer-checks -msoft-float -mbackchain