Could not allocate more than ~2GB with qemu-user
Description of problem:
On qemu-user, failed to allocate more than about 2GB on 32bit platform supporting up to 4GB (arm, ppc, etc.)
Steps to reproduce:
1. Try to allocate more than 2GB [e.g. for(i=0;i<64;i++) if(malloc(64*1024*1024)==NULL) perror("Failed to allocate 64MB");]
2. Only 1 64MB chunck is allocated in the upper 2GB memory space
3. Failed to allocate after about 2GB.
Additional information:
The problem is in **pageflags_find** and **pageflags_next** functions (found in _accel/tcg/user-exec.c_) 3rd parameters, that should be **target_ulong** instead of incorrect _target_long_ (the parameter will be converted signed extended to uint64_t).
The testing program is the following:
```
#include <stdio.h>
#include <stdlib.h>

int main(int argc,char *argv[]) {
  unsigned int a;
  unsigned int i;
  char *al;
  unsigned int sss=1U*1024*1024*64;
  for(a=0;a<128;a++) {
    al=malloc(sss);
    if(al!=NULL) {
      printf("ALLOC OK %u (%08lX)!\n",sss*(a+1),al);
    }
    else {
      printf("Cannot alloc %d\n",(a+1)*sss);
      perror("Cannot alloc");
      exit(1);
    }
  }
}
```