RISC-V: define _REENTRANT with -pthread

This is expected by the AX_PTHREAD autoconf macro from
<http://www.gnu.org/software/autoconf-archive/ax_pthread.html>.

	* config/riscv/linux.h (CPP_SPEC): Define.

From-SVN: r257621

gcc/ChangeLog

+2018-02-13  Andreas Schwab  <schwab@suse.de>
+
+	* config/riscv/linux.h (CPP_SPEC): Define.
+
 2018-02-13  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/84335

gcc/config/riscv/linux.h

@@ -47,6 +47,8 @@ along with GCC; see the file COPYING3.  If not see
 
 #define ICACHE_FLUSH_FUNC "__riscv_flush_icache"
 
+#define CPP_SPEC "%{pthread:-D_REENTRANT}"
+
 #define LINK_SPEC "\
 -melf" XLEN_SPEC "lriscv \
 %{shared} \