soc: riscv: riscv-privilege: add GP relative addressing support

Initialize GP at common entry point of riscv-privilege SoCs so that
this commit add GP support of these SoCs at once.

As some privilege SoCs want to customize the entry point, they should
disable linker relaxation by `.norelax option` before entrying the
common entry point `__start`.

Signed-off-by: Jim Shu <cwshu@andestech.com>