riscv: don't export local symbols

riscv: don't export local symbols

Symbols that are not used outside the file should not be exported
and should be declared as static.
Move the existing comments to the static declarations.

Change-Id: Idf208e3fda4b3f8df789553cf03ebf5f20d811bb
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7170
Reviewed-by: Jan Matyas <matyas@codasip.com>
Reviewed-by: Tim Newsome <tim@sifive.com>
Tested-by: jenkins