gcc: Drop gcc-cross-initial and use gcc-cross instead

gcc: Drop gcc-cross-initial and use gcc-cross instead

We need a libgcc to build glibc. Tranditionally we therefore build
a non-threaded and non-shared compiler (gcc-cross-initial), then use
that to build libgcc-initial which is used to build glibc which we can
then build gcc-cross and libgcc against.

Firstly, we can drop the glibc dependency from gcc-cross, *if* we make
two changes:

a) specify the minimum glibc version to support in a configure option
b) create a dummy limits.h file so that later when glibc creates one,
   the headers structure has support for it. We can do this with a simple
   empty file

Once gcc-cross is libc independent, we can use it to build both
libgcc-initial and then later libgcc.

libgcc-initial is tricky as we need to imitate the non-threaded and
non-shared case. We can do that by hacking the threading mode back to
"single" even if gcc reports "posix" and disable libc presence for the
libgcc-intial build. We have to create the dummy limits.h to avoid
compiler errors from a missing header.

glibc will fail to link with libgcc-initial due to a missing "exception
handler" capable libgcc (libgcc_eh.a). Since we know glibc doesn't need
any exception handler, we can safely symlink to libgcc.a.

With those changes, gcc-cross can be used in all places and we only need
one build of gcc for each architecture.

For some reason ifunc was being disabled on mips prior to these changes
but afterwards became enabled but caused assertion failures. This is
therefore disabled until we can debug that.

The glibc version is a minimum and only used on ppc in the gcc code, it
doesn't need to be rigerously kept exactly up to date.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>