riscv: fix wrong access width in assembly code

The thread->base.user_options field is an uint8_t. Access it using lb.
A "copy" of it is made into __esf.fp_state. Make that field an uint8_t
too and access it with lb/sb.

_callee_saved.fcsr is an uint32_t. Access it with lw/sw.
Ditto for is_user_mode.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>