LibCore: Add syscall wrapper for fork()

Userland/Libraries/LibCore/System.cpp

@@ -480,4 +480,12 @@ ErrorOr<void> mkdir(StringView path, mode_t mode)
 #endif
 }
 
+ErrorOr<pid_t> fork()
+{
+    pid_t pid = ::fork();
+    if (pid < 0)
+        return Error::from_syscall("fork"sv, -errno);
+    return pid;
+}
+
 }

Userland/Libraries/LibCore/System.h

@@ -64,5 +64,6 @@ ErrorOr<void> setegid(gid_t);
 ErrorOr<bool> isatty(int fd);
 ErrorOr<void> symlink(StringView target, StringView link_path);
 ErrorOr<void> mkdir(StringView path, mode_t);
+ErrorOr<pid_t> fork();
 
 }