fork: generate stub on no-MMU systems

fork() can be implemented using either the fork or clone syscalls on MMU
systems. Therefore the stub is only generated if neither __NR_fork nor
__NR_clone are defined. The stub code manually undefines __NR_fork on
no-MMU systems in an attempt to enable the stub, but this doesn't work
because __NR_clone is still defined. It is not appropriate to undefine
__NR_clone because clone is available on no-MMU, it is just not capable
of implementing fork.

This patch directly enables the fork stub if __ARCH_USE_MMU__ is not
defined. This eliminates the need to undefine __NR_fork, so this code is
removed

Signed-off-by: Ben Wolsieffer <ben.wolsieffer@hefring.com>

libc/sysdeps/linux/common/stubs.c

@@ -34,10 +34,6 @@ __attribute_used__ static int ret_enosys_stub(void)
 	link_warning(stub, #stub ": this function is not implemented") \
 	strong_alias(ret_enosys_stub, stub)
 
-#ifndef __ARCH_USE_MMU__
-# undef __NR_fork
-#endif
-
 #ifdef __arm__
 # define __NR_fadvise64_64 __NR_arm_fadvise64_64
 # define __NR_fadvise64 __NR_arm_fadvise64_64
@@ -120,7 +116,7 @@ make_stub(fgetxattr)
 make_stub(flistxattr)
 #endif
 
-#if !defined __NR_fork && !defined __NR_clone
+#if !defined __ARCH_USE_MMU__ || (!defined __NR_fork && !defined __NR_clone)
 make_stub(fork)
 #endif