m68k: Quiet up cppcheck warning [PR114689]

cppcheck apparently warns on the | !!sticky part of the expression and
using | (!!sticky) quiets it up (it is correct as is).
The following patch adds the ()s, and also adds them around mant >> 1 just
in case it makes it clearer to all readers that the expression is parsed
that way already.

2024-04-15  Jakub Jelinek  <jakub@redhat.com>

	PR libgcc/114689
	* config/m68k/fpgnulib.c (__truncdfsf2): Add parentheses around
	!!sticky bitwise or operand to quiet up cppcheck.  Add parentheses
	around mant >> 1 bitwise or operand.