Change the default cross compiler prefix to powerpc64le-linux-gnu-

That is what is used by the packaged cross-compilers on (at least)
Fedora and Ubuntu.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

README.md

@@ -16,7 +16,10 @@ You can try out Microwatt/Micropython without hardware by using the ghdl simulat
 
 - Build micropython. If you aren't building on a ppc64le box you
   will need a cross compiler. If it isn't available on your distro
-  grab the powerpc64le-power8 toolchain from https://toolchains.bootlin.com
+  grab the powerpc64le-power8 toolchain from https://toolchains.bootlin.com.
+  You may need to set the CROSS_COMPILE environment variable
+  to the prefix used for your cross compilers.  The default is
+  powerpc64le-linux-gnu-.
 
 ```
 git clone https://github.com/micropython/micropython.git

hello_world/Makefile

@@ -1,7 +1,7 @@
 ARCH = $(shell uname -m)
 ifneq ("$(ARCH)", "ppc64")
 ifneq ("$(ARCH)", "ppc64le")
-	CROSS_COMPILE ?= powerpc64le-linux-
+	CROSS_COMPILE ?= powerpc64le-linux-gnu-
 endif
 endif
 

rust_lib_demo/Makefile

@@ -1,7 +1,7 @@
 ARCH = $(shell uname -m)
 ifneq ("$(ARCH)", "ppc64")
 ifneq ("$(ARCH)", "ppc64le")
-	CROSS_COMPILE ?= powerpc64le-linux-
+	CROSS_COMPILE ?= powerpc64le-linux-gnu-
 endif
 endif
 

tests/Makefile.test

@@ -1,7 +1,7 @@
 ARCH = $(shell uname -m)
 ifneq ("$(ARCH)", "ppc64")
 ifneq ("$(ARCH)", "ppc64le")
-        CROSS_COMPILE ?= powerpc64le-linux-
+        CROSS_COMPILE ?= powerpc64le-linux-gnu-
         endif
         endif