Support _r_debug for static binaries.

We initialize _r_debug for static binaries to allows debug
agents to treat static binaries a little more like dyanmic
ones. This simplifies the work a debug agent has to do to
access TLS in a static binary via libthread_db.

Tested on x86_64.

See:
https://sourceware.org/ml/libc-alpha/2014-04/msg00183.html

	[BZ #16831]
	* csu/libc-start.c (LIBC_START_MAIN) [!SHARED]: Call
	_dl_debug_initialize.