lib/msun: Fix x86 GCC6 build after 221622ec0c8e184

Apparently GCC only supports arithmetic expressions that use static
const variables in initializers starting with GCC8. To keep older
versions happy use a macro instead.

Fixes: 221622ec0c ("lib/msun: Avoid FE_INEXACT for x86 log2l/log10l")
Reported by: Jenkins
Reviewed By: imp
Differential Revision: https://reviews.freebsd.org/D29233

(cherry picked from commit 05eac56a0432d07acd7f159125855437a4dd6259)