riscv: Initialize thread pointer before calling C functions

Because of the stack canary feature that reads from the current task
structure the stack canary value, the thread pointer register "tp" must
be set before calling any C function from head.S: by chance, setup_vm
and all the functions that it calls does not seem to be part of the
functions where the canary check is done, but in the following commits,
some functions will.

Fixes: f2c9699f ("riscv: Add STACKPROTECTOR supported")
Signed-off-by: Alexandre Ghiti <alexandre.ghiti@canonical.com>