arch: riscv: introduce global pointer relative addressing support

Enable RISC-V GP relative addressing by linker relaxation to reduce
the code size. It optimizes addressing of globals in small data section
(.sdata).

The gp initialization at program start needs each SoC support. Also,
if RISC-V SoC has custom linker script, SoC should provide
__global_pointer$ symbol in it's linker script.

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