mips: Fix non-POSIX regexp

When cross compiling a MIPS kernel on a BSD based HOSTCC leads
to errors like

  SYNC    include/config/auto.conf.cmd - due to: .config
egrep: empty (sub)expression
  UPD     include/config/kernel.release
  HOSTCC  scripts/dtc/dtc.o - due to target missing

It turns out that egrep uses this egrep pattern:

		(|MINOR_|PATCHLEVEL_)

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 egrep implementation
while a BSD host complains.

Such patterns can be replaced by a transformation like

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

Fixes: 48c35b2d ("[MIPS] There is no __GNUC_MAJOR__")
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

arch/mips/Makefile

@@ -321,7 +321,7 @@ KBUILD_LDFLAGS		+= -m $(ld-emul)
 
 ifdef CONFIG_MIPS
 CHECKFLAGS += $(shell $(CC) $(KBUILD_CFLAGS) -dM -E -x c /dev/null | \
-	egrep -vw '__GNUC_(|MINOR_|PATCHLEVEL_)_' | \
+	egrep -vw '__GNUC_(MINOR_|PATCHLEVEL_)?_' | \
 	sed -e "s/^\#define /-D'/" -e "s/ /'='/" -e "s/$$/'/" -e 's/\$$/&&/g')
 endif