x86-64: strlen-evex performance performance degradation compared to strlen-avx2

[abush wang]

Hi, H.J.
When I test glibc performance between 2.28 and 2.38,
I found there is a performance degradation about strlen.
In fact, this difference comes from __strlen_avx2 and __strlen_evex

```
2.28
__strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:42
42 ENTRY (STRLEN)


2.38
__strlen_evex () at ../sysdeps/x86_64/multiarch/strlen-evex.S:79
79 ENTRY_P2ALIGN (STRLEN, 6)
```

This is my test:
```
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>

#define MAX_STRINGS 100

uint64_t rdtsc() {
    uint32_t lo, hi;
    __asm__ __volatile__ (
        "rdtsc" : "=a"(lo), "=d"(hi)
    );
    return ((uint64_t)hi << 32) | lo;
}

int main(int argc, char *argv[]) {
    char *input_str[MAX_STRINGS];
    size_t lengths[MAX_STRINGS];
    int num_strings = 0; // Number of input strings
    uint64_t start_cycles, end_cycles;

    // Parse command line arguments and store pointers in input_str array
    for (int i = 1; i < argc && num_strings < MAX_STRINGS; ++i) {
        input_str[num_strings] = argv[i];
        num_strings++;
    }

    // Measure the strlen operation for each string
    start_cycles = rdtsc();
    for (int i = 0; i < num_strings; ++i) {
        lengths[i] = strlen(input_str[i]);
    }
    end_cycles = rdtsc();

    unsigned long long total_cycle = end_cycles - start_cycles;
    unsigned long long av_cycle = total_cycle / num_strings;

    // Print the total cycles taken for the strlen operations
    printf("Total cycles: %llu av cycle: %llu \n", total_cycle, av_cycle);

    // Print the recorded lengths
    printf("Lengths of the input strings:\n");
    for (int i = 0; i < num_strings; ++i) {
        printf("String %d length: %zu\n", i, lengths[i]);
    }

    return 0;
}
```

This is result
```
2.28
./strlen_test str1 str2 str3 str4 str5
Total cycles: 1468 av cycle: 293
Lengths of the input strings:
String 0 length: 4
String 1 length: 4
String 2 length: 4
String 3 length: 4
String 4 length: 4

2.38
./strlen_test str1 str2 str3 str4 str5
Total cycles: 1814 av cycle: 362
Lengths of the input strings:
String 0 length: 4
String 1 length: 4
String 2 length: 4
String 3 length: 4
String 4 length: 4
```

Thanks,
abush
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