[libc] Fix long double is double double const (#113258)
Turns out for double double LDBL_MANT_DIG == 106. This patch fixes the
constant. Should fix the ppc buildbot.
Previously:
https://github.com/llvm/llvm-project/pull/113235
https://github.com/llvm/llvm-project/issues/113237
https://github.com/llvm/llvm-project/pull/91651