package/gcc: define _REENTRANT for RISC-V when -pthread is passed

The detection of pthread support fails on RISC-V unless _REENTRANT is
defined. This commit backports a patch that is already in upstream gcc
8.x to gcc 7.x to define _REENTRANT when -pthread is passed.

This will replace a number of package-specific fixes that have been
introduced to define _REENTRANT.

Signed-off-by: Mark Corbin <mark.corbin@embecosm.com>
[Thomas: tweak commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/gcc/7.3.0/1001-gcc-define-_REENTRANT-for-RISC-V-when-pthread-is-pas.patch

@@ -0,0 +1,30 @@
+From 6ea832d09415cd82b744d0cf168cdd728c43df47 Mon Sep 17 00:00:00 2001
+From: Mark Corbin <mark.corbin@embecosm.com>
+Date: Thu, 22 Nov 2018 12:19:11 +0000
+Subject: [PATCH] gcc: define _REENTRANT for RISC-V when -pthread is passed
+
+The detection of pthread support fails on RISC-V unless _REENTRANT
+is defined. Added the CPP_SPEC definition from gcc 8.1.0 to correct
+this.
+
+Signed-off-by: Mark Corbin <mark.corbin@embecosm.com>
+---
+ gcc/config/riscv/linux.h | 2 ++
+ 1 file changed, 2 insertions(+)
+
+diff --git a/gcc/config/riscv/linux.h b/gcc/config/riscv/linux.h
+index 4b2f7b6e1fd..b00d23ddfa0 100644
+--- a/gcc/config/riscv/linux.h
++++ b/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} \
+-- 
+2.19.1
+