[PowerPC] Fix feholdexcept() on powerpc referencing the wrong struct member

In r355656, endianness handling of the floating point environment was fixed
in the PowerPC code to work as intended.

However, one bit got missed, causing feholdexcept() to mis-save the fenv.

Submitted by: Renato Riolino <renato.riolino@eldorado.org.br>
Differential Revision: https://reviews.freebsd.org/D23382