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.

testdata/recover.txt

@@ -49,3 +49,7 @@ outer recovered: repanic value
   recovered: nil map
   recovered: closed chan
   recovered: close nil chan
+
+# recover from hardware signals
+  recovered: divide by zero
+  recovered: nil pointer dereference