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 <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/[e-mail]

arch/powerpc/Makefile

···
 # Rewritten by Cort Dougan and Paul Mackerras
 #
 
+ifdef cross_compiling
+  ifeq ($(CROSS_COMPILE),)
+    # Auto detect cross compiler prefix.
+    # Look for: (powerpc(64(le)?)?)(-unknown)?-linux(-gnu)?-
+    CC_ARCHES := powerpc powerpc64 powerpc64le
+    CC_SUFFIXES := linux linux-gnu unknown-linux-gnu
+    CROSS_COMPILE := $(call cc-cross-prefix, $(foreach a,$(CC_ARCHES), \
+                       $(foreach s,$(CC_SUFFIXES),$(a)-$(s)-)))
+  endif
+endif
+
 HAS_BIARCH	:= $(call cc-option-yn, -m32)
 
 # Set default 32 bits cross compilers for vdso and boot wrapper