linux-user: incorrect alignment of sigframe::pretcode & rt_sigframe::pretcode cause crash
Description of problem:
Corrent Print Result:

sp: cdd3b4e8

SUCCEEDED!

qemu-x86_64 Print Result:

sp: 2804170

qemu: uncaught target signal 11 (Segmentation fault) - core dumped

Segmentation fault

Reason of Bug:

sigframe::pretcode & rt_sigframe::pretcode must align of 16n-sizeof(void*) instead of 16n, Because rsp align of 16n before instruction "call" in caller, After "call", push address of "call" in caller. sp of begin in callee is 16n-sizeof(void*)

For example on x86_64:

reference to "qemu/linux-user/i386/signal.c"

```
# define TARGET_FPSTATE_FXSAVE_OFFSET 0

struct rt_sigframe {
    abi_ulong pretcode;
    struct target_ucontext uc;
    struct target_siginfo info;
    struct target_fpstate fpstate QEMU_ALIGNED(16);
};
#define TARGET_RT_SIGFRAME_FXSAVE_OFFSET (                                 \
    offsetof(struct rt_sigframe, fpstate) + TARGET_FPSTATE_FXSAVE_OFFSET)
```

offsetof(struct rt_sigframe, fpstate) align of 16

TARGET_FPSTATE_FXSAVE_OFFSET is 0

TARGET_RT_SIGFRAME_FXSAVE_OFFSET is 16n, also alignment of fxsave is 64

so address of rt_sigframe::pretcode is 16n instead of 16n - sizeof(void*), It is incorect!

Fix the bug:

```
struct rt_sigframe {
    abi_ulong pretcode;
    struct target_ucontext uc;
    struct target_siginfo info;
    abi_ulong unused QEMU_ALIGNED(16);
    struct target_fpstate fpstate;
};
```

offsetof(struct rt_sigframe, fpstate) is 16n+8, so address of rt_sigframe::pretcode is 16n-8 on x86_64.