META_MODE: Pass along the default sysroot in bootstrap-tools to avoid rebuilds later.

Some of the clang libraries build in this phase and the cross-tools
phase. Later in the cross-tools phase when they build with a default
TOOLS_PREFIX, they see a changed build command in meta mode due to
the changed DEFAULT_SYSROOT. This is avoided by passing along
TOOLS_PREFIX earlier.

Sponsored by: EMC / Isilon Storage Division