rtld-elf/riscv: Don't allocate static TLS for dynamic TLS relocations

Provided you don't run out of extra static TLS space this should work,
but it's wholly unnecessary and not how things are supposed to be done.
Only static TLS relocations should allocate static TLS.

Reviewed by: kib
Fixes: 4b1859c0e943 ("Add support for RISC-V architecture.")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D50563