Building AIX cross-compiler requires -bbigtoc

[David Edelsohn <dje at watson dot ibm dot com>]

	Nearly all of the GCC backends require the -bbigtoc flag to
overcome TOC overflow when they are linked on AIX.  When building a native
compiler, BOOT_LDFLAGS takes care of this during bootstrapping.  When
building a cross compiler, BOOT_LDFLAGS is never used.

	Because XLC requires -bbigtoc and GCC requires -Wl,-bbigtoc, the
compiler being used for the build determines what option is required.
However, languages other than C need GCC, so GCC would seem to be a
requirements for building a cross-compiler.

	It would be nice if building the cross-compiler always used
BOOT_LDFLAGS so that using the correct LDFLAGS would be automatic, but I
do not see any convenient place to enforce that substitution.  Any
suggestions?

Thanks, David