riscv: replace use of callee-saved register in standalone

riscv: replace use of callee-saved register in standalone

Register x19 (s3) is a callee-saved register. It must not be used to
load and jump to exported functions without saving it beforehand.
Replace it with t0, a temporary and caller-saved register.

Change the code comment to reflect this and fix it to correctly list gp
as the register with the pointer to global data.

Signed-off-by: Lukas Auer <[email protected]>
Reviewed-by: Bin Meng <[email protected]>
Tested-by: Bin Meng <[email protected]>