runtime: make divide-by-zero and nil dereference panics recoverable

Modify the Unix signal handler to redirect execution to a Go sigpanic
function instead of printing an error and re-raising the signal.
The C signal handler modifies the ucontext to make the faulting
instruction appear to have called tinygo_sigpanic, which then calls
runtimePanic with the appropriate message.

Supported on all architectures TinyGo targets on Linux and Darwin:
x86_64, i386, aarch64, ARM, and MIPS. Also registers SIGFPE, which
was previously not handled at all.

src/runtime/os_darwin.go

@@ -24,6 +24,7 @@ const (
 // https://opensource.apple.com/source/xnu/xnu-7195.141.2/bsd/sys/signal.h.auto.html
 const (
 	sig_SIGBUS  = 10
+	sig_SIGFPE  = 8
 	sig_SIGILL  = 4
 	sig_SIGSEGV = 11
 )