riscv: align the heap to 16 bytes

This may be expected by the ABI. In particular, it means that stacks
allocated on the heap will be 16-byte aligned.

targets/riscv.ld

@@ -58,7 +58,7 @@ SECTIONS
 }
 
 /* For the memory allocator. */
-_heap_start = _ebss;
+_heap_start = ALIGN(_ebss, 16);
 _heap_end = ORIGIN(RAM) + LENGTH(RAM);
 _globals_start = _sdata;
 _globals_end = _ebss;