Redundant floating point rounding mode store/restore

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119832

--- Comment #2 from Li Pan  ---
More details.

  7 --- mode change from 10 -> 7  // NONE => DYN   <<<
  8 --- restore mode is dyn and prev is not call 
  9 --- mode change from 7 -> 9   // DYN => CALL
 10 --- mode change from 10 -> 9  // NONE => CALL
 11 --- mode change from 9 -> 7   // CALL => DYN   <<<
 12 --- backup from prev is call ---
 13 --- mode change from 10 -> 9  // NONE => CALL
 14 --- mode change from 9 -> 7   // CALL => DYN   <<<
 15 --- backup from prev is call ---
 16 --- mode change from 7 -> 9   // DYN => CALL
 17 --- mode change from 7 -> 8   // DYN => EXIT

When prev mode is CALL, will always backup frm, and then we meet NONE => DYN,
as kito mentioned, the restore will be emitted if mode is dyn and prev mode is
not CALL.  Not very sure if we can take care of this safely inside the
riscv_emit_frm_mode_set right now. Let me reconsider this.