[libc][obvious] fix sqrt when long double is double

Previously, the "fsqrt" instruction was used on all x86_64 platforms
for finding the square root of long doubles. On long double is double
platforms (e.g. windows) this created errors. This patch changes square
root function for long doubles to be the same as the one for doubles if
long doubles are doubles.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D122688