add __libc_current_sigrtmin and __libc_current_sigrtmax

libc/build.mak

@@ -348,6 +348,9 @@ libc += multibyte/wcstombs.o
 libc += multibyte/wctob.o
 libc += multibyte/wctomb.o
 
+libc += signal/sigrtmax.o
+libc += signal/sigrtmin.o
+
 libc += stdio/__fclose_ca.o
 libc += stdio/__fdopen.o
 libc += stdio/__fmodeflags.o

libc/manifest.txt

@@ -11,6 +11,7 @@ All the files under the following directories are imported from musl:
   math
   misc
   multibyte
+  signal
   stdio
   stdlib
   temp

libc/signal/sigrtmax.c

+int __libc_current_sigrtmax()
+{
+	return 64;
+}

libc/signal/sigrtmin.c

+int __libc_current_sigrtmin()
+{
+	return 35;
+}