runtime: include <syscall.h> and <sys/syscall.h> if available

Fixes Solaris build.
    
    Reviewed-on: https://go-review.googlesource.com/c/162885

From-SVN: r268940

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-a9c1a76e14b66a356d3c3dfb50f1e6138e97733c
+6877c95a5f44c3ab4f492d2000ce07771341d7b7
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/go-varargs.c

@@ -12,6 +12,12 @@
 #include <sys/types.h>
 #include <fcntl.h>
 #include <sys/ioctl.h>
+#ifdef HAVE_SYSCALL_H
+#include <syscall.h>
+#endif
+#ifdef HAVE_SYS_SYSCALL_H
+#include <sys/syscall.h>
+#endif
 
 /* The syscall package calls C functions.  The Go compiler can not
    represent a C varargs functions.  On some systems it's important