Usermode exec-stack fix

When loading a shared library that requires an executable stack,
glibc uses the mprotext PROT_GROWSDOWN flag to achieve this.
We don't support PROT_GROWSDOWN.
Add a special case to handle changing the stack permissions in this way.

Signed-off-by: Paul Brook <paul@codesourcery.com>

linux-user/qemu.h

@@ -42,6 +42,7 @@ struct image_info {
         abi_ulong       mmap;
         abi_ulong       rss;
         abi_ulong       start_stack;
+        abi_ulong       stack_limit;
         abi_ulong       entry;
         abi_ulong       code_offset;
         abi_ulong       data_offset;