The WebAssembly target seems to have been designed to be always passed a
--sysroot, likely because of being tested to work only with the
WASI-SDK. This results into passing bare, non-existing paths in include
paths, such as:
-internal-isystem /include/wasm32-wasi -internal-isystem /include
(and similar for /lib/).
In Debian, the wasi-libc package ships its files in
/usr/include/wasm32-wasi, /usr/lib/wasm32-wasi etc.
Add support in the target for including paths from /usr as well. To
avoid changing the code in more intrusive ways (to do e.g. what the
Linux target does) add a bit of an indirection where the "sysroot"
defaults to "/usr" instead of the empty string. This should probably be
adjusted a bit if it were to be upstreamed.
Closes: #1020746
