PR #73725 introduced the in-process plugin server library, but the
selection of the library depends on the selected toolchain, which
depends on the compiler target, not the host. When cross-compiling (for
example from macOS to a embedded Unix target), the compiler will
incorrectly chose the `.so` file, not find it, and fail to compile
things like the `@debugDescription` macro.
Move the in-process plugin server library code from the platform
toolchains into the parent type, and code it so it uses the right name
depending on the compiler host at compilation time. This discards the
target and only relies on the compiler host for selecting the right
library.
