s390/decompressor: fix misaligned symbol build error

s390/decompressor: fix misaligned symbol build error

Nathan Chancellor reported a kernel build error on Fedora 39:

$ clang --version | head -1
clang version 16.0.5 (Fedora 16.0.5-1.fc39)

$ s390x-linux-gnu-ld --version | head -1
GNU ld version 2.40-1.fc39

$ make -skj"$(nproc)" ARCH=s390 CC=clang CROSS_COMPILE=s390x-linux-gnu- olddefconfig all
s390x-linux-gnu-ld: arch/s390/boot/startup.o(.text+0x5b4): misaligned symbol `_decompressor_end' (0x35b0f) for relocation R_390_PC32DBL
make[3]: *** [.../arch/s390/boot/Makefile:78: arch/s390/boot/vmlinux] Error 1

It turned out that the problem with misaligned symbols on s390 was fixed
with commit 80ddf5ce1c92 ("s390: always build relocatable kernel") for the
kernel image, but did not take into account that the decompressor uses its
own set of CFLAGS, which come without -fPIE.

Add the -fPIE flag also to the decompresser CFLAGS to fix this.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: CKI <cki-project@redhat.com>
Suggested-by: Ulrich Weigand <Ulrich.Weigand@de.ibm.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/1747
Link: https://lore.kernel.org/32935.123062114500601371@us-mta-9.us.mimecast.lan/
Link: https://lore.kernel.org/r/20230622125508.1068457-1-hca@linux.ibm.com
Cc: <stable@vger.kernel.org>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>