compiler: Rename host_build() to tools_build()

With the new TOOLS_LIBCRYPTO and some other changes, it seems that we are
heading towards calling this a tools build rather than a host build,
although of course it does happen on the host.

I cannot think of anything built by the host which cannot be described as
a tool, so rename this function.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>