Introduce target feature crt_static_allows_dylibs

Most UNIX-like platforms do not allow shared libraries to statically
link their own libc, as libc expects to have consistent process-global
state. On those platforms, when we do not have a shared libc available,
we must not attempt to link dylibs or cdylibs. On Windows, however, it
is expected to statically link the CRT into dynamic libraries.

This feature is only relevant for targets that support both fully-static
and fully-dynamic linkage, such as musl on Linux.

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