Simplify the signalhandlers module (#854)

* Remove the unused EnsureDarwinMachPorts function

When compiling the C++ shims for longjmp/setjmp/signal handling we don't
use the `USE_APPLE_MACH_PORTS` directive, so this function was entirely
unused anyway. This looks to be a holdover from when this was originally
copied from C++, but no need for keeping around this now-legacy
initialization.

* Remove the `wasmtime_init_finish` function

This looks like it's perhaps largely historical cruft at this point now
I think? The function, with the removal of the mach ports from the
previous commit, only reads the initializtion state of the signal
handlers. If the signal handlers failed to get installed, though, it
simply returns early rather than erroring out anyway. In any case a
follow-up commit will refactor `wasmtime_init_eager` to handle this as
well.

* Pare down `wasmtime_init_eager`

Similar to previous commits it looks like this function may have accrued
some debt over time, nowadays it doesn't really do much other than
capture a backtrace and install signal handlers. The `lazy_static` state
isn't really that necessary and we can rely on the `Once` primitive in
the standard library for one-time initialization.

This also updates the code to unconditionally panic if signal handlers
fail to get installed, which I think is the behavior that we'll want for
now and we can enhance it over time if necessary, but I don't think we
have a use case where it's currently necessary.

crates/runtime/src/lib.rs

@@ -43,7 +43,6 @@ pub use crate::instance::{InstanceHandle, InstantiationError, LinkError};
 pub use crate::jit_int::GdbJitImageRegistration;
 pub use crate::mmap::Mmap;
 pub use crate::sig_registry::SignatureRegistry;
-pub use crate::signalhandlers::{wasmtime_init_eager, wasmtime_init_finish};
 pub use crate::trap_registry::{get_mut_trap_registry, get_trap_registry, TrapRegistrationGuard};
 pub use crate::traphandlers::{wasmtime_call, wasmtime_call_trampoline, Trap};
 pub use crate::vmcontext::{