Revert "arch: riscv: added support for custom initialization of gp register"

This reverts commit 7b09d031fae3969fbe8647a57190f1fa21fabd8e. Because
context save of GP register is removed, we don't need to initialize GP
at thread init. GP will be a constant value so that it could only be
initialized at program start.

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