linux-user: Define AT_RANDOM to support target stack protection mechanism.

The dynamic linker from the GNU C library v2.10+ uses the ELF
auxiliary vector AT_RANDOM [1] as a pointer to 16 bytes with random
values to initialize the stack protection mechanism.  Technically the
emulated GNU dynamic linker crashes due to a NULL pointer
derefencement if it is built with stack protection enabled and if
AT_RANDOM is not defined by the QEMU ELF loader.

[1] This ELF auxiliary vector was introduced in Linux v2.6.29.

This patch can be tested with the code above:

    #include <elf.h>       /* Elf*_auxv_t, AT_RANDOM, */
    #include <stdio.h>     /* printf(3), */
    #include <stdlib.h>    /* exit(3), EXIT_*, */
    #include <stdint.h>    /* uint8_t, */
    #include <string.h>    /* memcpy(3), */

    #if defined(__LP64__) || defined(__ILP64__) || defined(__LLP64__)
    #    define Elf_auxv_t Elf64_auxv_t
    #else
    #    define Elf_auxv_t Elf32_auxv_t
    #endif

    main(int argc, char* argv[], char* envp[])
    {
        Elf_auxv_t *auxv;

        /* *envp = NULL marks end of envp. */
        while (*envp++ != NULL);

        /* auxv->a_type = AT_NULL marks the end of auxv. */
        for (auxv = (Elf_auxv_t *)envp; auxv->a_type != AT_NULL; auxv++) {
            if (auxv->a_type == AT_RANDOM) {
                int i;
                uint8_t rand_bytes[16];

                printf("AT_RANDOM is: 0x%x\n", auxv->a_un.a_val);
                memcpy(rand_bytes, (const uint8_t *)auxv->a_un.a_val, sizeof(rand_bytes));
                printf("it points to: ");
                for (i = 0; i < 16; i++) {
                    printf("0x%02x ", rand_bytes[i]);
                }
                printf("\n");
                exit(EXIT_SUCCESS);
            }
        }
        exit(EXIT_FAILURE);
    }

Changes introduced in v2 and v3:

    * Fix typos + thinko (AT_RANDOM is used for stack canary, not for
      ASLR)

    * AT_RANDOM points to 16 random bytes stored inside the user
      stack.

    * Add a small test program.

Signed-off-by: Cédric VINCENT <cedric.vincent@st.com>
Signed-off-by: Laurent ALFONSI <laurent.alfonsi@st.com>
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
