[riscv] Use load and store pseudo-instructions where possible

[riscv] Use load and store pseudo-instructions where possible

The pattern of "load address to register" followed by "load value from
address in register" generally results in three instructions: two to
load the address and one to load the value.

This can be reduced to two instructions by allowing the assembler to
incorporate the low bits of the address within the load (or store)
instruction itself.  In the case of a store, this requires specifying
a second register that can be temporarily used to hold the high bits
of the address.  (In the case of a load, the destination register is
reused for this purpose.)

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