[riscv] Place prefix debug strings in .rodata

[riscv] Place prefix debug strings in .rodata

The GNU assembler does not seem to automatically assume alignment to
an instruction boundary for sections containing assembled code.

Place the prefix debug strings (if present) in .rodata rather than in
.prefix, to avoid potentially creating misaligned code sections.

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