msun: swap words order instead of bits order on BIG ENDIAN

The "for" loop on big endian was inverting all the bits instead of
just the words

Issue reported by TestSuite (msun lib nan_test case)

Submitted by: Renato Riolino <renato.riolino@eldorado.org.br>
Submitted by: Fernando Valle <fernando.valle@eldorado.org.br>
Reviewed by: pfg, alfredo
Approved by: jhibbits (mentor)
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D23926