Don't export _end on arm64 and riscv.

These platforms don't support brk() and sbrk(), which are the reason
for exporting _end in the first place.

MFC after:	1 week

lib/libc/aarch64/Symbol.map

@@ -33,6 +33,5 @@ FBSD_1.0 {
 
 FBSDprivate_1.0 {
 	_set_tp;
-	_end;
 	__makecontext;
 };

lib/libc/riscv/Symbol.map

@@ -33,6 +33,5 @@ FBSD_1.0 {
 
 FBSDprivate_1.0 {
 	_set_tp;
-	_end;
 	__makecontext;
 };