riscv: Change to use positive offset to access relocation entries

The codes currently skip the very first relocation entry, and have
an inaccurate comment "skip first reserved entry" indicating that
the first entry is reserved, but later it references the elements
in the first relocation entry using a minus offset.

Change to use a positive offset so that there is no need to skip
the first relocation entry.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>