powerpc/Makefile: Auto detect cross compiler

If no cross compiler is specified, try to auto detect one.

Look for various combinations, matching:
  powerpc(64(le)?)?(-unknown)?-linux(-gnu)?-

There are more possibilities, but the above is known to find a compiler
on Fedora and Ubuntu (which use linux-gnu-), and also detects the
kernel.org cross compilers (which use linux-).

This allows cross compiling with simply:

 # Ubuntu
 $ sudo apt install gcc-powerpc-linux-gnu
 # Fedora
 $ sudo dnf install gcc-powerpc64-linux-gnu

 $ make ARCH=powerpc defconfig
 $ make ARCH=powerpc -j 4

Inspired by arch/parisc/Makefile.

Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231206115548.1466874-4-mpe@ellerman.id.au