tsan: fix syscall test on aarch64

[dvyukov]

Summary

Add missing includes and use SYS_pipe2 instead of SYS_pipe
as it's not present on some arches.

compiler-rt/test/tsan/Linux/syscall.h

+#include <fcntl.h>
 #include <sanitizer/linux_syscall_hooks.h>
+#include <signal.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
 #include <unistd.h>
 
 int myfork() {
   __sanitizer_syscall_pre_fork();
 #ifdef SYS_fork
   int res = syscall(SYS_fork);
 #else
   int res = syscall(SYS_clone, SIGCHLD, 0);
 #endif
   __sanitizer_syscall_post_fork(res);
   return res;
 }
 
 int mypipe(int pipefd[2]) {
   __sanitizer_syscall_pre_pipe(pipefd);
-  int res = syscall(SYS_pipe, pipefd);
+  int res = syscall(SYS_pipe2, pipefd, 0);
   __sanitizer_syscall_post_pipe(res, pipefd);
   return res;
 }
 
 int myclose(int fd) {
   __sanitizer_syscall_pre_close(fd);
   int res = syscall(SYS_close, fd);
   __sanitizer_syscall_post_close(res, fd);