linux-user: Honor PT_GNU_STACK

Map the stack executable if required by default or on demand.

Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index 7d90de1..e2e93fb 100644
--- a/linux-user/qemu.h
+++ b/linux-user/qemu.h
@@ -48,6 +48,7 @@
         uint32_t        elf_flags;
         int             personality;
         abi_ulong       alignment;
+        bool            exec_stack;
 
         /* Generic semihosting knows about these pointers. */
         abi_ulong       arg_strings;   /* strings for argv */