qemu: signal correct exit code to QEMU

There were a few issues that were causing qemu-system-arm and
qemu-system-riscv to give the wrong exit codes. They are in fact capable
of exiting with 0 or 1 signalled from the running application, but this
functionality wasn't used. This commit changes this in the following
ways:

  * It fixes SemiHosting codes, which were incorrectly written in
    decimal while they should have been written in hexadecimal (oops!).
  * It modifies all the baremetal main functions (aka reset handlers) to
    exit with `exit(0)` instead of `abort()`.
  * It changes `syscall.Exit` to call `exit(code)` instead of `abort()`
    on baremetal targets.
  * It adds these new exit functions where necessary, implemented in a
    way that signals the correct exit status if running under QEMU.

All in all, this means that `tinygo test` doesn't have to look at the
output of a test to determine the outcome. It can simply look at the
exit code.

src/runtime/runtime_fe310_qemu.go

@@ -21,6 +21,16 @@ func nanosecondsToTicks(ns int64) timeUnit {
 }
 
 func abort() {
-	// Signal a successful exit.
-	testExit.Set(0x5555)
+	exit(1)
+}
+
+func exit(code int) {
+	if code == 0 {
+		// Signal a successful exit.
+		testExit.Set(0x5555) // FINISHER_PASS
+	} else {
+		// Signal a failure. The exit code is stored in the upper 16 bits of the
+		// 32 bit value.
+		testExit.Set(uint32(code)<<16 | 0x3333) // FINISHER_FAIL
+	}
 }