386: bump minimum requirement to the Pentium 4

Previously we used the i386 target, probably with all optional features
disabled. However, the Pentium 4 has been released a _long_ time ago and
it seems reasonable to me to take that as a minimum requirement.

Upstream Go now also seems to move in this direction:
https://github.com/golang/go/issues/40255

The main motivation for this is that there were floating point issues
when running the tests for the math package:

    GOARCH=386 tinygo test math

I haven't investigated what's the issue, but I strongly suspect it's
caused by the weird x87 80-bit floating point format. This could perhaps
be fixed in a different way (by setting the FPU precision to 64 bits)
but I figured that just setting the minimum requirement to the Pentium 4
would probably be fine. If needed, we can respect the GO386 environment
variable to support these very old CPUs.

To support this newer CPU, I had to make sure that the stack is aligned
to 16 bytes everywhere. This was not yet always the case.

compileopts/target.go

@@ -253,6 +253,9 @@ func defaultTarget(goos, goarch, triple string) (*TargetSpec, error) {
 		GDB:              []string{"gdb"},
 		PortReset:        "false",
 	}
+	if goarch == "386" {
+		spec.CPU = "pentium4"
+	}
 	if goos == "darwin" {
 		spec.CFlags = append(spec.CFlags, "-isysroot", "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk")
 		spec.LDFlags = append(spec.LDFlags, "-Wl,-dead_strip")