[AArch64] Add optimized strchr.

Implementation of strchr for AArch64.  Speedups taken from micro-bench
show the improvements relative to the standard C code.

The use of LD1 means we have identical code for both big- and
little-endian systems.
Remove unnecessary $(.)

The variable is not necessary, especially since it does not exist.
malloc/malloc.c: Avoid calling sbrk unnecessarily with zero

Due to my bad review suggestion for the fix for BZ #15089 a check
was removed from systrim to prevent sbrk being called with a zero
argument. Add the check back to avoid this useless work.

ChangeLog:

2014-06-19  Will Newton  <will.newton@linaro.org>

	* malloc/malloc.c (systrim): If extra is zero then return
	early.
Fix memory leak in regexp compiler (BZ #17069)
Add x86_64 memset optimized for AVX2

In this patch we take advantage of HSW memory bandwidth, manage to
reduce miss branch prediction by avoiding using branch instructions and
force destination to be aligned with avx & avx2 instruction.

The CPU2006 403.gcc benchmark indicates this patch improves performance
from 26% to 59%.

	* sysdeps/x86_64/multiarch/Makefile: Add memset-avx2.
	* sysdeps/x86_64/multiarch/memset-avx2.S: New file.
	* sysdeps/x86_64/multiarch/memset.S: Likewise.
	* sysdeps/x86_64/multiarch/memset_chk.S: Likewise.
	* sysdeps/x86_64/multiarch/rtld-memset.S: Likewise.
Use 3 bytes for __pad1 in pthread_rwlock_t for x32

Since long is 4 bytes for x32, we should use 3 bytes for __pad1 when
a long __pad1 is replaced by a byte __rwelision and __pad1.

	* sysdeps/x86/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Use
	3 bytes for __pad1 for x32.
	(__PTHREAD_RWLOCK_ELISION_EXTRA): Likewise.