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 for cases where lo == 0by defining a constant and using that.