RISC-V: Enable static-pie.

This patch referents the commit 374cef32 to add static-pie support. And
because the dummy link map is used when relocating ourselves, so need
not to set __global_pointer$ at this time.

It will also check whether toolchain supports to build static-pie.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>