* for non libc.so libraries, don't forget to link libc.so

* avoid link errors on linker-provided symbols.
  i feel this is a workaround for a wasm-ld bug.

when building a shared library, specify all CFLAGS because
it can contain options like -resource-dir.

In the wasi-sdk windows job, when building libc.so, for some reasons,
the -isystem option in CFLAGS here seems to prevent the linker from
finding compiler-rt.

I don't understand how it happens because:

* Afaik, -isystem should not affect how the linker search objects

* The compiler-rt path is explicitly given as BUILTINS_LIB

Anyway, this commit tries to work it around by passing only a subset
of the options.