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.

src/internal/task/task_stack_386.go

@@ -16,6 +16,9 @@ type calleeSavedRegs struct {
 	ebp uintptr
 
 	pc uintptr
+
+	// Pad this struct so that tasks start on a 16-byte aligned stack.
+	_ [3]uintptr
 }
 
 // archInit runs architecture-specific setup for the goroutine startup.