tools/nolibc: x86-64: Use `mov $60,%eax` instead of `mov $60,%rax`

Note that mov to 32-bit register will zero extend to 64-bit register.
Thus `mov $60,%eax` has the same effect with `mov $60,%rax`. Use the
shorter opcode to achieve the same thing.
```
  b8 3c 00 00 00       	mov    $60,%eax (5 bytes) [1]
  48 c7 c0 3c 00 00 00 	mov    $60,%rax (7 bytes) [2]
```
Currently, we use [2]. Change it to [1] for shorter code.

Signed-off-by: Ammar Faizi <ammar.faizi@students.amikom.ac.id>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>