RISC-V: Fix an alignment warning in libthr

Compiling with clang gives a loss-of-alignment error due the cast to
uint8_t *. Since the TLS is always tcb aligned and TP_OFFSET is defined
as sizeof(struct tcb) we can guarantee there is no misalignment. Silence
the error by moving the offset into the inline assembly.

Reviewed by: br
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D21926