Add support for uclibc NPTL toolchain.

This patch modifies current toolchain build sequence so that
NPTL enabled toolchain can be built. The new sequence works
well with linuxthreads as well.

It introduces a new pass for gcc cross compilation. The new
sequence is binutils->gcc-initial->linux-headers -> uclibc-configured
(some cheats to generate phony shared libc.so and libm.o)
-> gcc-intermediate(with shared lib support) -> uclibc -> gcc-final

I also added a new sample config arm_nptl_toolchain_defconfig which
builds the toolchain and busybox.

I have only tried it on arm. However it should work for other
architectures which support NPTL on uclibc e.g. mips, sh, x86, ppc, x86_64

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>