sysinfo.c: ignore ptrace_peeksiginfo_args from <linux/ptrace.h>

With some versions of glibc and GNU/Linux ptrace_pseeksiginfo_args is
defined in both <sys/ptrace.h> and <linux/ptrace.h>. We don't actually
care about the struct, so use a #define to avoid a redefinition error.

This fixes https://gcc.gnu.org/PR81324.

Change-Id: Ide04efe871da21f1a55f646a8f42a4f4283d9c50
Reviewed-on: https://go-review.googlesource.com/49290
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/libgo/sysinfo.c b/libgo/sysinfo.c
index 1ba27b1..884d017 100644
--- a/libgo/sysinfo.c
+++ b/libgo/sysinfo.c
@@ -106,9 +106,13 @@
 /* Avoid https://sourceware.org/bugzilla/show_bug.cgi?id=762 .  */
 #define ia64_fpreg pt_ia64_fpreg
 #define pt_all_user_regs pt_ia64_all_user_regs
+/* Avoid redefinition of ptrace_peeksiginfo from <sys/ptrace.h>.
+   https://gcc.gnu.org/PR81324 .  */
+#define ptrace_peeksiginfo_args ignore_ptrace_peeksiginfo_args
 #include <linux/ptrace.h>
 #undef ia64_fpreg
 #undef pt_all_user_regs
+#undef ptrace_peeksiginfo_args
 #endif
 #if defined(HAVE_LINUX_RTNETLINK_H)
 #include <linux/rtnetlink.h>