Wrong result for strncmp-avx2 and strncmp-evex when it hits the page cross case followed by the loop page cross case immediately afterwards with a short length.

See following test case:

```
#include <assert.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <wchar.h>

#define PAGE_SIZE 4096
static void *
make_buf(uint64_t sz) {
    void * p = mmap(NULL, 2 * PAGE_SIZE + sz, PROT_NONE,
                    MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
    assert(p != NULL);
    assert(!mprotect(p + PAGE_SIZE, sz, PROT_READ | PROT_WRITE));
    return p + PAGE_SIZE;
}

int
main(int argc, char ** argv) {
    char * s1 = make_buf(0xd000 + 0x4000);
    char * s2 = make_buf(0xd000 + 0x4000);

    s1 += 0xffa;
    s2 += 0xfed;

    strcpy(s1, "tst-tlsmod%");
    strcpy(s2, "tst-tls-manydynamic73mod");

    if (strncmp(s1, s2, 10) != 0) {
        printf("Error!\n");
    }
}
```


Issue (for both evex and avx2)

```
	.p2align 4,, 10
L(return_page_cross_end_check):
	tzcntl	%ecx, %ecx
```

`ecx` can have garbabe in it from before the start of the string.

Fix is to use the `r10` mask to and off these bad matches (strcmp was doing
so. It was an oversight not to to do so in strncmp).