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.