Fix HOST_TOOLCHAIN_PREFIX
Fix HOST_TOOLCHAIN_PREFIX

Previously HOST_TOOLCHAIN_PREFIX can't accept toolchain in arch-os-*-gcc
format.  Fix it so we can try out new host toolchain, eg.

  HOST_TOOLCHAIN_PREFIX=prebuilts/gcc/linux-x86/host/x86_64-linux-glibc2.11-4.6/bin/x86_64-linux- make

Change-Id: Ic1092593036c41d5471e788654fb4e0991dd7e40

xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build