nptl: Use out-of-line wake function in __libc_lock_unlock slow path

This slightly reduces code size, as can be seen below.
__libc_lock_unlock is usually used along with __libc_lock_lock in
the same function.  __libc_lock_lock already has an out-of-line
slow path, so this change should not introduce many additional
non-leaf functions.

This change also fixes a link failure in 32-bit Arm thumb mode
because commit 1f9c804f
("nptl: Use internal low-level lock type for !IS_IN (libc)")
introduced __libc_do_syscall calls outside of libc.

Before x86-64:

   text	   data	    bss	    dec	    hex	filename
1937748	  20456	  54896	2013100	 1eb7ac	libc.so.6
  25601	    856	  12768	  39225	   9939	nss/libnss_db.so.2
  40310	    952	  25144	  66406	  10366	nss/libnss_files.so.2

After x86-64:
   text	   data	    bss	    dec	    hex	filename
1935312	  20456	  54896	2010664	 1eae28	libc.so.6
  25559	    864	  12768	  39191	   9917	nss/libnss_db.so.2
  39764	    960	  25144	  65868	  1014c	nss/libnss_files.so.2

Before i686:

2110961	  11272	  39144	2161377	 20fae1	libc.so.6
  27243	    428	  12652	  40323	   9d83	nss/libnss_db.so.2
  43062	    476	  25028	  68566	  10bd6	nss/libnss_files.so.2

After i686:

2107347	  11272	  39144	2157763	 20ecc3	libc.so.6
  26929	    432	  12652	  40013	   9c4d	nss/libnss_db.so.2
  43132	    480	  25028	  68640	  10c20	nss/libnss_files.so.2

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>