gh-124932: Distinguish build prefix from host prefix in cross builds (#124933)

In Emscripten and other cross builds, the build file system and the host file
system look different. For instance, we may want to install into
`cross-build/$TARGET/lib`, and then mount that as `/lib` in the host file
system. This change adds a distinction between:

* `prefix` -- the path in the build file system where we want to install the files
* `host_prefix` -- the path in the host file system where getpath.c will look for the files

And similarly for `exec_prefix` and `host_exec_prefix`. At present, this is only
used for Emscripten.