Undo _map_libc mapping of arc4random* functions (re: 13c6f5d)

It breaks the build on (at least) Void (musl) and 32-bit Slackware
15.0 (glibc-2.33). The libast dependencies are too intricate and
messy for it to be reasonable to fix this properly, so just undo.
Thanks to @pghvlaans for the report.

Resolves: #778

src/lib/libast/Mamfile

@@ -138,12 +138,6 @@ make install virtual
 		exec - invoke_iffe %{<}
 	done
 
-	note * ast_random.h *
-	make FEATURE/random
-		makp features/random
-		exec - invoke_iffe %{<}
-	done
-
 	note * sig.h: AST replacement for <signal.h>
 	make FEATURE/sig
 		makp features/sig.sh
@@ -193,7 +187,6 @@ make install virtual
 		make FEATURE/map
 			make features/map.c
 				prev sig.h
-				prev FEATURE/random
 				prev FEATURE/api
 				prev FEATURE/eaccess
 				prev FEATURE/options
@@ -583,6 +576,12 @@ make install virtual
 		exec - invoke_iffe %{<} -I.
 	done
 
+	note * ast_random.h *
+	make FEATURE/random
+		makp features/random
+		exec - invoke_iffe %{<}
+	done
+
 	note * copy most of the FEATURE/* results above to canonically named header files; separating
 	note * this allows all of the above tests, which don't depend on each other, to run in parallel
 

src/lib/libast/features/map.c

@@ -30,7 +30,6 @@
 #include "FEATURE/options"
 #include "FEATURE/eaccess"
 #include "FEATURE/api"
-#include "FEATURE/random"
 #include <sig.h>
 
 #if _opt_map_libc && !defined(_map_libc)
@@ -67,20 +66,6 @@ main(void)
 	printf("\n");
 	printf("#define	_map_libc	1\n");
 #endif
-#if _map_libc
-#if !_lib_arc4random
-	printf("#undef	arc4random\n");
-	printf("#define arc4random	_ast_arc4random\n");
-#endif
-#if !_lib_arc4random_buf
-	printf("#undef	arc4random_buf\n");
-	printf("#define arc4random_buf	_ast_arc4random_buf\n");
-#endif
-#if !_lib_arc4random_uniform
-	printf("#undef	arc4random_uniform\n");
-	printf("#define arc4random_uniform	_ast_arc4random_uniform\n");
-#endif
-#endif
 #if _map_libc || defined(__linux__)
 	printf("#undef	basename\n");
 	printf("#define basename	_ast_basename\n");