Support six-argument syscalls from C for 32-bit x86, use generic lowlevellock-futex...

Support six-argument syscalls from C for 32-bit x86, use generic lowlevellock-futex.h (bug 18138).

This patch follows the approach outlined in
<https://sourceware.org/ml/libc-alpha/2015-03/msg00656.html> to
support six-argument syscalls from INTERNAL_SYSCALL for 32-bit x86,
making them call a function __libc_do_syscall that takes the syscall
number and three syscall arguments in the registers in which the
kernel expects them, along with a pointer to a structure containing
the other three arguments.

In turn, this allows the generic lowlevellock-futex.h to be used on
32-bit x86, so supporting lll_futex_timed_wait_bitset (and so allowing
FUTEX_CLOCK_REALTIME to be used in various cases, so fixing bug 18138
for 32-bit x86 and leaving hppa as the only architecture missing
lll_futex_timed_wait_bitset).  The change to lowlevellock.h's
definition of SYS_futex is because the generic lowlevelloc-futex.h
ends up bringing in bits/syscall.h which defines SYS_futex to
__NR_futex, so resulting in redefinition errors.  The revised
definition in lowlevellock.h is in line with what the x86_64 version
does.

__libc_do_syscall is only needed in libpthread at present (meaning
nothing special needs to be done to make it shared-only in most
libraries containing it, static in libc only, as on ARM).

Tested for 32-bit x86, with the glibc testsuite and with the test in
bug 18138.  The failures seen

FAIL: nptl/tst-cleanupx4
FAIL: rt/tst-cpuclock2

are pre-existing.

	[BZ #18138]
	* sysdeps/unix/sysv/linux/i386/sysdep.h (struct
	libc_do_syscall_args): New structure.
	(INTERNAL_SYSCALL_MAIN_0): New macro.
	(INTERNAL_SYSCALL_MAIN_1): Likewise.
	(INTERNAL_SYSCALL_MAIN_2): Likewise.
	(INTERNAL_SYSCALL_MAIN_3): Likewise.
	(INTERNAL_SYSCALL_MAIN_4): Likewise.
	(INTERNAL_SYSCALL_MAIN_5): Likewise.
	(INTERNAL_SYSCALL_MAIN_6): Likewise.  Call __libc_do_syscall.
	(INTERNAL_SYSCALL): Define to use INTERNAL_SYSCALL_MAIN_##nr.
	Replace conditional definitions by conditional definitions of ....
	(INTERNAL_SYSCALL_MAIN_INLINE): ... this.  New macro.
	* sysdeps/unix/sysv/linux/i386/libc-do-syscall.S: New file.
	* sysdeps/unix/sysv/linux/i386/Makefile [$(subdir) = nptl]
	(libpthread-sysdep_routines): Add libc-do-syscall.
	* sysdeps/unix/sysv/linux/i386/lowlevellock-futex.h: Remove file.
	* sysdeps/unix/sysv/linux/i386/lowlevellock.h (SYS_futex): Define
	to __NR_futex not 240.