Partially revert r271349, which disabled the msun cexp test #4 on i386.

Ensure the expected result is stored first in a volatile variable with
the desired type. This makes all the tests succeed.

Slightly changed from the original pull request, but functionally the
same.

Obtained from: https://github.com/freebsd/freebsd/pull/401
Submitted by: Moritz Buhl <gh@moritzbuhl.de>
PR: 191676
MFC after: 3 days