linux-user: Fix structure target_ucontext for MIPS

linux-user: Fix structure target_ucontext for MIPS

Structure ucontext for MIPS is defined in the following way in
Linux kernel:

(arch/mips/include/uapi/asm/ucontext.h, lines 54-64)

struct ucontext {
    /* Historic fields matching asm-generic */
    unsigned long       uc_flags;
    struct ucontext     *uc_link;
    stack_t             uc_stack;
    struct sigcontext   uc_mcontext;
    sigset_t            uc_sigmask;

    /* Extended context structures may follow ucontext */
    unsigned long long	uc_extcontext[0];
};

Fix the structure target_ucontext for MIPS to reflect the definition
above, except the correction for field uc_extcontext, which will
follow at some later time.

Fixes: 94c5495d

Reported-by: Dragan Mladjenovic <dmladjenovic@wavecomp.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <1562931470-3700-2-git-send-email-aleksandar.markovic@rt-rk.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>