[riscv] Place explicitly zero-initialised variables in the .data section

[riscv] Place explicitly zero-initialised variables in the .data section

Variables in the .bss section cannot be relied upon to have zero
values during early initialisation, before we have relocated ourselves
to somewhere suitable in RAM and zeroed the .bss section.

Place any explicitly zero-initialised variables in the .data section
rather than in .bss, so that we can rely on their values even during
this early initialisation stage.

Signed-off-by: Michael Brown <mcb30@ipxe.org>