[builtins] Fix __floattitf and __floatuntitf on x86

These conversion functions were using LDBL_MANT_DIG (which is the 80-bit
extended float on x86) instead of the appropriate macro for the 128-bit
floating point type expected by the *tf* softfloat library calls.
This was discovered while testing D98261 (which allows building the *tf*
functions on x86).

This also changes the constants used in the two tests to use 128-bit
floating-point literals instead of long double ones to ensure that the
comparison succeeds on platforms with smaller long double (e.g. x86_64)

Reviewed By: scanon

Differential Revision: https://reviews.llvm.org/D131787