gcc/configure.ac: Fix cross build by using $(CFLAGS-$@) [PR100598]

BUILD_CFLAGS is set by configure; by default, BUILD_CFLAGS = $(ALL_CFLAGS)
is used. The latter contains (see gcc/Makefile.in) $(CFLAGS-$@), which is used
to pass .o-file specific flags to the compiler.
For cross builds, BUILD_CFLAGS is constructed in configure{,.ac} and missed
the $(CFLAGS-$@) - despite the comment above ALL_CFLAGS that configure.ac
might have to kept in sync.

gcc/ChangeLog:

	PR other/100598
	* configure: Regenerate.
	* configure.ac (BUILD_CFLAG, BUILD_CXXFLAGS): Add $(CFLAGS-$@).