Makefile: Search for distro-provided cross-compiler

Search for different variations of cross-compiler prefixes that might be in
the PATH, in this order of preference:

1. powerpc64-linux-gcc
2. powerpc64le-linux-gcc
3. powerpc64-linux-gnu-gcc
4. powerpc64le-linux-gnu-gcc

The latter two are available as distro packages in at least Debian, Fedora
and Ubuntu.

Tested with GNU Make 3.82 (CentOS 7) and GNU make 4.2.1 (Fedora 30).

Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Tested-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>

Makefile

@@ -16,7 +16,15 @@ ifdef CROSS_COMPILE
 endif
 ifneq ("$(ARCH)", "ppc64")
 ifneq ("$(ARCH)", "ppc64le")
+ifneq ($(shell which powerpc64-linux-gcc 2> /dev/null),)
 	CROSS ?= powerpc64-linux-
+else ifneq ($(shell which powerpc64le-linux-gcc 2> /dev/null),)
+	CROSS ?= powerpc64le-linux-
+else ifneq ($(shell which powerpc64-linux-gnu-gcc 2> /dev/null),)
+	CROSS ?= powerpc64-linux-gnu-
+else ifneq ($(shell which powerpc64le-linux-gnu-gcc 2> /dev/null),)
+	CROSS ?= powerpc64le-linux-gnu-
+endif
 endif
 endif