nak: fix frnd on SM50
Previously we weren't setting bit 42 when encoding frnd as f2f. This
causes it to ignore the rounding mode when src_type == dst_type
rather than using it for integer rounding.
Tested against dEQP-VK.glsl.builtin.precision.{ceil,trunc,floor,round}.*
, which were failing before the change and passing after.
Edited by Olivia Lee