lib/msun: Avoid FE_INEXACT for x86 log2l/log10l

[arichardson]

Summary

This fixes tests/lib/msun/logarithm_test after compiling the test with
-fno-builtin (D28577). Adding invln10_lo + invln10_10 results in
FE_INEXACT and the same for the log2l invln2_lo + invln2_hi. This patch
avoids FE_INEXACT by defining a constant and using that.

Test Plan

logarithm_test works for me now.

[arichardson]

Simpler approach

[dim]

Although the original authors seem to dislike constant folding by the compiler, it is a good idea here. :)