x86 setup: correct segfault in generation of 32-bit reloc kernel

Impact: segfault on build of a 32-bit relocatable kernel

When converting arch/x86/boot/compressed/relocs.c to support unlimited
sections, the computation of sym_strtab in walk_relocs() was done
incorrectly.  This causes a segfault for some people when building the
relocatable 32-bit kernel.

Pointed out by Anonymous <pageexec@freemail.hu>.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>