jit: avoid calloc() poisoning on musl [PR106102]

On musl <pthread.h> uses calloc() (via <sched.h>). jit/ includes
it directly and exposes use of poisoned calloc():

    /build/build/./prev-gcc/xg++ ... ../../gcc-13-20220626/gcc/jit/jit-playback.cc
    make[3]: *** [Makefile:1143: jit/libgccjit.o] Error 1
    make[3]: *** Waiting for unfinished jobs....
    In file included from /<<NIX>>/musl-1.2.3-dev/include/pthread.h:30,
                     from ../../gcc-13-20220626/gcc/jit/jit-playback.cc:44:
    /<<NIX>>/musl-1.2.3-dev/include/sched.h:84:7: error: attempt to use poisoned "calloc"
       84 | void *calloc(size_t, size_t);
          |       ^
    /<<NIX>>/musl-1.2.3-dev/include/sched.h:124:36: error: attempt to use poisoned "calloc"
      124 | #define CPU_ALLOC(n) ((cpu_set_t *)calloc(1,CPU_ALLOC_SIZE(n)))
          |                                    ^

The change moves <pthread.h> inclusion to "system.h" under new
INCLUDE_PTHREAD_H guard and uses this mechanism in libgccjit.

gcc/

	PR c++/106102
	* system.h: Introduce INCLUDE_PTHREAD_H macros to include <pthread.h>.

gcc/jit/

	PR c++/106102
	* jit-playback.cc: Include <pthread.h> via "system.h" to avoid calloc()
	poisoning.
	* jit-recording.cc: Ditto.
	* libgccjit.cc: Ditto.

(cherry picked from commit 49d5080)

gcc/jit/jit-playback.cc

@@ -19,6 +19,7 @@ along with GCC; see the file COPYING3.  If not see
 <http://www.gnu.org/licenses/>.  */
 
 #include "config.h"
+#define INCLUDE_PTHREAD_H
 #include "system.h"
 #include "coretypes.h"
 #include "target.h"
@@ -41,8 +42,6 @@ along with GCC; see the file COPYING3.  If not see
 #include "diagnostic.h"
 #include "stmt.h"
 
-#include <pthread.h>
-
 #include "jit-playback.h"
 #include "jit-result.h"
 #include "jit-builtins.h"

gcc/jit/jit-recording.cc

@@ -19,13 +19,13 @@ along with GCC; see the file COPYING3.  If not see
 <http://www.gnu.org/licenses/>.  */
 
 #include "config.h"
+#define INCLUDE_PTHREAD_H
 #include "system.h"
 #include "coretypes.h"
 #include "tm.h"
 #include "pretty-print.h"
 #include "toplev.h"
 
-#include <pthread.h>
 
 #include "jit-builtins.h"
 #include "jit-recording.h"

gcc/jit/libgccjit.cc

@@ -19,12 +19,12 @@ along with GCC; see the file COPYING3.  If not see
 <http://www.gnu.org/licenses/>.  */
 
 #include "config.h"
+#define INCLUDE_PTHREAD_H
 #include "system.h"
 #include "coretypes.h"
 #include "timevar.h"
 #include "typed-splay-tree.h"
 #include "cppbuiltin.h"
-#include <pthread.h>
 
 #include "libgccjit.h"
 #include "jit-recording.h"

gcc/system.h

@@ -750,6 +750,10 @@ extern int vsnprintf (char *, size_t, const char *, va_list);
 #endif
 #endif
 
+#ifdef INCLUDE_PTHREAD_H
+#include <pthread.h>
+#endif
+
 #ifdef INCLUDE_ISL
 #ifdef HAVE_isl
 #include <isl/options.h>