arch/risc-v: introduce dynamic stack allocation.

It is misleading to allocate stack from static array and heap,
make all stack allocated from heap area.

Signed-off-by: Inochi Amaoto <inochiama@outlook.com>

boards/risc-v/k230/canmv230/scripts/ld-flat.script

@@ -114,6 +114,7 @@ SECTIONS
       *(.sbss.*)
       *(.gnu.linkonce.b*)
       *(COMMON)
+      . = ALIGN(32);
       _ebss = . ;
     }