std.c: Fix sigrtmin()/sigrtmax() for FreeBSD and NetBSD.

They just define the constants in the system headers.

Author: alexrp

lib/std/c.zig

@@ -10385,12 +10385,20 @@ pub const sigaction = switch (native_os) {
 
 /// Zig's version of SIGRTMIN.  Actually a function.
 pub fn sigrtmin() u8 {
-    return @truncate(@as(c_uint, @bitCast(private.__libc_current_sigrtmin())));
+    return switch (native_os) {
+        .freebsd => 65,
+        .netbsd => 33,
+        else => @truncate(@as(c_uint, @bitCast(private.__libc_current_sigrtmin()))),
+    };
 }
 
 /// Zig's version of SIGRTMAX.  Actually a function.
 pub fn sigrtmax() u8 {
-    return @truncate(@as(c_uint, @bitCast(private.__libc_current_sigrtmax())));
+    return switch (native_os) {
+        .freebsd => 126,
+        .netbsd => 63,
+        else => @truncate(@as(c_uint, @bitCast(private.__libc_current_sigrtmax()))),
+    };
 }
 
 pub const sigfillset = switch (native_os) {