riscv: Set text_offset correctly for M-Mode

M-Mode Linux is loaded at the start of RAM, not 2MB later. Perhaps this
should be calculated based on PAGE_OFFSET somehow? Even better would be to
deprecate text_offset and instead introduce something absolute.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>