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")

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.

src/runtime/arch_386.go

@@ -9,7 +9,7 @@ const TargetBits = 32
 
 // Align on word boundary.
 func align(ptr uintptr) uintptr {
-	return (ptr + 3) &^ 3
+	return (ptr + 15) &^ 15
 }
 
 func getCurrentStackPointer() uintptr {

src/runtime/gc_386.S

@@ -13,10 +13,11 @@ tinygo_scanCurrentStack:
     pushl %ebp
 
     // Scan the stack.
+    subl $8, %esp // adjust the stack before the call to maintain 16-byte alignment
     pushl %esp
     calll tinygo_scanstack
 
     // Restore the stack pointer. Registers do not need to be restored as they
     // were only pushed to be discoverable by the GC.
-    addl $20, %esp
+    addl $28, %esp
     retl

src/runtime/math.go

@@ -217,7 +217,7 @@ func math_sinh(x float64) float64
 
 //go:linkname math_Sqrt math.Sqrt
 func math_Sqrt(x float64) float64 {
-	if GOARCH == "x86" || GOARCH == "amd64" || GOARCH == "wasm" {
+	if GOARCH == "386" || GOARCH == "amd64" || GOARCH == "wasm" {
 		return llvm_sqrt(x)
 	}
 	return math_sqrt(x)