x86/tools/relocs: Fix non-POSIX regexp

Trying to run a cross-compiled x86 relocs tool on a BSD based
HOSTCC leads to errors like

  VOFFSET arch/x86/boot/compressed/../voffset.h - due to: vmlinux
  CC      arch/x86/boot/compressed/misc.o - due to: arch/x86/boot/compressed/../voffset.h
  OBJCOPY arch/x86/boot/compressed/vmlinux.bin - due to: vmlinux
  RELOCS  arch/x86/boot/compressed/vmlinux.relocs - due to: vmlinux
empty (sub)expressionarch/x86/boot/compressed/Makefile:118: recipe for target 'arch/x86/boot/compressed/vmlinux.relocs' failed
make[3]: *** [arch/x86/boot/compressed/vmlinux.relocs] Error 1

It turns out that relocs.c uses patterns like

	"something(|_end)"

This is not valid syntax or gives undefined results according
to POSIX 9.5.3 ERE Grammar

	https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html

It seems to be silently accepted by the Linux regexp() implementation
while a BSD host complains.

Such patterns can be replaced by a transformation like

	"(|p1|p2)" -> "(p1|p2)?"

Fixes: fd952815

 ("x86-32, relocs: Whitelist more symbols for ld bug workaround")
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>