Add .sbss section to .bss section

This is a special section that RISC-V uses (see the comment in
the Tock board linker)

layout_generic.ld

diff --git a/layout_generic.ld b/layout_generic.ld
index fde1c40..6b96ee5 100644
--- a/layout_generic.ld
+++ b/layout_generic.ld
@@ -122,7 +122,7 @@
     {
         . = ALIGN(4); /* Make sure we're word-aligned here */
         _bss = .;
-        KEEP(*(.bss*))
+        KEEP(*(.bss* .sbss*))
         *(COMMON)
         . = ALIGN(4);
     } > SRAM