[ARC] Define _REENTRANT when -pthread is passed.

The compiler is supposed to have the builtin defined _REENTRANT defined
when -pthread is passed, which wasn't done on the ARC architecture.

When _REENTRANT is not passed, the C library will not use reentrant
functions, and the latest version of ax_pthread.m4 from the
autoconf-archive will no longer detect that thread support is
available (see https://savannah.gnu.org/patch/?8186

).

gcc/
2017-03-28  Claudiu Zissulescu  <claziss@synopsys.com>
	    Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

	* config/arc/arc.h (CPP_SPEC): Add subtarget_cpp_spec.
	(EXTRA_SPECS): Define.
	(SUBTARGET_EXTRA_SPECS): Likewise.
	(SUBTARGET_CPP_SPEC): Likewise.
	* config/arc/elf.h (EXTRA_SPECS): Renamed to
	SUBTARGET_EXTRA_SPECS.
	* config/arc/linux.h (SUBTARGET_CPP_SPEC): Define.

Co-Authored-By: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

From-SVN: r246524