runtime: remove extalloc

The extalloc collector has been broken for a while, and it doesn't seem reasonable to fix right now.
In addition, after a recent change it no longer compiles.
In the future similar functionality can hopefully be reintroduced, but for now this seems to be the most reasonable option.

src/runtime/runtime_unix.go

@@ -1,3 +1,4 @@
+//go:build (darwin || (linux && !baremetal && !wasi)) && !nintendoswitch
 // +build darwin linux,!baremetal,!wasi
 // +build !nintendoswitch
 
@@ -200,13 +201,6 @@ func syscall_Exit(code int) {
 	exit(code)
 }
 
-func extalloc(size uintptr) unsafe.Pointer {
-	return malloc(size)
-}
-
-//export free
-func extfree(ptr unsafe.Pointer)
-
 // TinyGo does not yet support any form of parallelism on an OS, so these can be
 // left empty.
 
@@ -217,3 +211,46 @@ func procPin() {
 //go:linkname procUnpin sync/atomic.runtime_procUnpin
 func procUnpin() {
 }
+
+var heapSize uintptr = 128 * 1024 // small amount to start
+var heapMaxSize uintptr
+
+var heapStart, heapEnd uintptr
+
+func preinit() {
+	// Allocate a large chunk of virtual memory. Because it is virtual, it won't
+	// really be allocated in RAM. Memory will only be allocated when it is
+	// first touched.
+	heapMaxSize = 1 * 1024 * 1024 * 1024 // 1GB for the entire heap
+	for {
+		addr := mmap(nil, heapMaxSize, flag_PROT_READ|flag_PROT_WRITE, flag_MAP_PRIVATE|flag_MAP_ANONYMOUS, -1, 0)
+		if addr == unsafe.Pointer(^uintptr(0)) {
+			// Heap was too big to be mapped by mmap. Reduce the maximum size.
+			// We might want to make this a bit smarter than simply halving the
+			// heap size.
+			// This can happen on 32-bit systems.
+			heapMaxSize /= 2
+			continue
+		}
+		heapStart = uintptr(addr)
+		heapEnd = heapStart + heapSize
+		break
+	}
+}
+
+// growHeap tries to grow the heap size. It returns true if it succeeds, false
+// otherwise.
+func growHeap() bool {
+	if heapSize == heapMaxSize {
+		// Already at the max. If we run out of memory, we should consider
+		// increasing heapMaxSize on 64-bit systems.
+		return false
+	}
+	// Grow the heap size used by the program.
+	heapSize = (heapSize * 4 / 3) &^ 4095 // grow by around 33%
+	if heapSize > heapMaxSize {
+		heapSize = heapMaxSize
+	}
+	setHeapEnd(heapStart + heapSize)
+	return true
+}