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.go

@@ -44,6 +44,9 @@ func main() {
 
 	println("\n# recover from nil map and closed channel")
 	recoverNilMapAndChan()
+
+	println("\n# recover from hardware signals")
+	recoverSignals()
 }
 
 func recoverSimple() {
@@ -261,3 +264,15 @@ func recoverNilMapAndChan() {
 		close(ch)
 	})
 }
+
+// Test recovering from hardware signals (SIGFPE, SIGSEGV).
+func recoverSignals() {
+	recoverMustPanic("divide by zero", func() {
+		var x int
+		println(1 / x)
+	})
+	recoverMustPanic("nil pointer dereference", func() {
+		var p *int
+		println(*p)
+	})
+}

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