Add support for custom Linux host toolchains when building the SDK.

This is needed in order to build Linux SDK binaries that can run
properly on Ubuntu 8.04 (Hardy). By default, the host toolchain
on 10.04 (Lucid) generates machine code that won't run on Hardy
due to GLibc ABI mistmatches.

Note that nothing happens if the new toolchain is not in the
prebuilt tree.

Change-Id: I914f5a303f16b6871759ce5a7178585ed3060870