Simplify memcpy() and memmove() on the x86-64 arch.

The x86-64 arch has a 'rep movsb' instruction, which can perform
memcpy() using only a single instruction, given:

    %rdi = destination
    %rsi = source
    %rcx = length

Additionally, it can also handle the overlapping case by setting DF=1
(backward copy), which can be used as the memmove() implementation.

Before this patch:
```
  00000000000010ab <memmove>:
    10ab: 48 89 f8              mov    %rdi,%rax
    10ae: 31 c9                 xor    %ecx,%ecx
    10b0: 48 39 f7              cmp    %rsi,%rdi
    10b3: 48 83 d1 ff           adc    $0xffffffffffffffff,%rcx
    10b7: 48 85 d2              test   %rdx,%rdx
    10ba: 74 25                 je     10e1 <memmove+0x36>
    10bc: 48 83 c9 01           or     $0x1,%rcx
    10c0: 48 39 f0              cmp    %rsi,%rax
    10c3: 48 c7 c7 ff ff ff ff  mov    $0xffffffffffffffff,%rdi
    10ca: 48 0f 43 fa           cmovae %rdx,%rdi
    10ce: 48 01 cf              add    %rcx,%rdi
    10d1: 44 8a 04 3e           mov    (%rsi,%rdi,1),%r8b
    10d5: 44 88 04 38           mov    %r8b,(%rax,%rdi,1)
    10d9: 48 01 cf              add    %rcx,%rdi
    10dc: 48 ff ca              dec    %rdx
    10df: 75 f0                 jne    10d1 <memmove+0x26>
    10e1: c3                    ret

  00000000000010e2 <memcpy>:
    10e2: 48 89 f8              mov    %rdi,%rax
    10e5: 48 85 d2              test   %rdx,%rdx
    10e8: 74 12                 je     10fc <memcpy+0x1a>
    10ea: 31 c9                 xor    %ecx,%ecx
    10ec: 40 8a 3c 0e           mov    (%rsi,%rcx,1),%dil
    10f0: 40 88 3c 08           mov    %dil,(%rax,%rcx,1)
    10f4: 48 ff c1              inc    %rcx
    10f7: 48 39 ca              cmp    %rcx,%rdx
    10fa: 75 f0                 jne    10ec <memcpy+0xa>
    10fc: c3                    ret
```

After this patch:
```
  // memmove is an alias for memcpy
  000000000040133b <memcpy>:
    40133b: 48 89 d1              mov    %rdx,%rcx
    40133e: 48 89 f8              mov    %rdi,%rax
    401341: 48 89 fa              mov    %rdi,%rdx
    401344: 48 29 f2              sub    %rsi,%rdx
    401347: 48 39 ca              cmp    %rcx,%rdx
    40134a: 72 03                 jb     40134f <memcpy+0x14>
    40134c: f3 a4                 rep movsb %ds:(%rsi),%es:(%rdi)
    40134e: c3                    ret
    40134f: 48 8d 7c 0f ff        lea    -0x1(%rdi,%rcx,1),%rdi
    401354: 48 8d 74 0e ff        lea    -0x1(%rsi,%rcx,1),%rsi
    401359: fd                    std
    40135a: f3 a4                 rep movsb %ds:(%rsi),%es:(%rdi)
    40135c: fc                    cld
    40135d: c3                    ret
```

v3:
  - Make memmove as an alias for memcpy (Willy).
  - Make the forward copy the likely case (Alviro).

v2:
  - Fix the broken memmove implementation (David).

Link: https://lore.kernel.org/lkml/20230902062237.GA23141@1wt.eu
Link: https://lore.kernel.org/lkml/5a821292d96a4dbc84c96ccdc6b5b666@AcuMS.aculab.comSuggested-by: David Laight <David.Laight@aculab.com>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>