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/arch_amd64.go

@@ -12,6 +12,7 @@ const callInstSize = 5 // "call someFunction" is 5 bytes
 const (
 	linux_MAP_ANONYMOUS = 0x20
 	linux_SIGBUS        = 7
+	linux_SIGFPE        = 8
 	linux_SIGILL        = 4
 	linux_SIGSEGV       = 11
 )