Kernel/LibC: add free function for FixedWidthAllocator

I have to rework the syscall API and allocators in process. For
now this works well enough :)

libc/stdlib.cpp

@@ -79,7 +79,9 @@ void* calloc(size_t nmemb, size_t size)
 	return ptr;
 }
 
-void free(void*)
+void free(void* ptr)
 {
-
+	if (ptr == nullptr)
+		return;
+	syscall(SYS_FREE, ptr);
 }

libc/unistd.cpp

@@ -74,6 +74,12 @@ long syscall(long syscall, ...)
 			ret = Kernel::syscall(SYS_ALLOC, bytes);
 			break;
 		}
+		case SYS_FREE:
+		{
+			void* ptr = va_arg(args, void*);
+			ret = Kernel::syscall(SYS_FREE, ptr);
+			break;
+		}
 		default:
 			puts("LibC: Unhandeled syscall");
 	}