i386/bits/syscalls.h: generate better code using more restrictive asm constraints

i386/bits/syscalls.h: generate better code using more restrictive asm constraints

Example: time() doesn't push/pop two registers now, and is smaller:

    <__GI_time>:
    -57                     push   %edi
    -8b 7c 24 08            mov    0x8(%esp),%edi
    -53                     push   %ebx
    -89 fb                  mov    %edi,%ebx
    +8b 4c 24 04            mov    0x4(%esp),%ecx
    +87 cb                  xchg   %ecx,%ebx
     b8 0d 00 00 00         mov    $0xd,%eax
     cd 80                  int    $0x80
    -5b                     pop    %ebx
    -5f                     pop    %edi
    +87 cb                  xchg   %ecx,%ebx
     c3                     ret

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>