[riscv] Split out runtime relocator to libprefix.S

[riscv] Split out runtime relocator to libprefix.S

Split out the runtime relocation logic from sbiprefix.S to a new
library libprefix.S.

Since this logically decouples the process of runtime relocation from
the _sbi_start symbol (currently used to determine the base address
for applying relocations), provide an alternative mechanism for the
relocator to determine the base address.

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