runtime: add new functions total_memory/0 and free_memory/0 (#18499)

vlib/runtime/free_memory_impl_darwin.c.v

@@ -0,0 +1,34 @@
+module runtime
+
+#include <mach/mach.h>
+
+[typedef]
+struct C.vm_size_t {
+}
+
+[typedef]
+struct C.vm_statistics64_data_t {
+	free_count          u32
+	purgeable_count     u32
+	speculative_count   u32
+	external_page_count u32
+}
+
+fn C.mach_host_self() C.host_t
+fn C.host_page_size(host C.host_t, out_page_size &C.vm_size_t) int
+fn C.host_statistics64(host C.host_t, flavor int, host_info_out &int, host_info_outCnt &u32) int
+
+fn free_memory_impl() usize {
+	$if macos {
+		mut hs := C.vm_statistics64_data_t{}
+		mut vmsz := u32(C.HOST_VM_INFO64_COUNT)
+		mut hps := u32(0)
+		mut host := C.mach_host_self()
+		unsafe {
+			C.host_statistics64(host, C.HOST_VM_INFO64, &int(&hs), &vmsz)
+			C.host_page_size(host, &C.vm_size_t(&hps))
+		}
+		return usize(u64(hs.free_count) * u64(hps))
+	}
+	return 1
+}