Remove configure test for ARM TLS descriptors support.

GCC 4.7 added support for ARM TLS descriptors.  The binutils support
is present in binutils 2.22.  Thus, this patch removes the associated
configure test as obsolete (leaving Makefile conditionals, as NaCl
sets have-arm-tls-desc = no in its sysdeps fragment).

Tested for ARM that installed shared libraries are unchanged by the
patch.

	* sysdeps/arm/configure.ac (libc_cv_arm_tls_desc): Remove
	configure test.
	* sysdeps/arm/configure: Regenerated.
	* sysdeps/arm/Makefile [!have-arm-tls-desc] (have-arm-tls-desc):
	Define variable if not already defined.