Change default BUILD_ARCH from ppc64 to ppc64le
Change default BUILD_ARCH from ppc64 to ppc64le

This really only has an effect when building the cross compiler
(when the HOST_ARCH is an x86 variant), and the build architecture
is not specified.

Signed-off-by: Paul A. Clarke <793d6d7c60bd6329d6f91fdb8c1b53aab99bf3da@us.ibm.com>

rzinsly/advance-toolchain,advancetoolchain/advance-toolchain,rzinsly/advance-toolchain,rzinsly/advance-toolchain,advancetoolchain/advance-toolchain,rzinsly/advance-toolchain,rzinsly/advance-toolchain,advancetoolchain/advance-toolchain,advancetoolchain/advance-toolchain,rzinsly/advance-toolchain,advancetoolchain/advance-toolchain,advancetoolchain/advance-toolchain