[compiler-rt] Implement __extendxftf2 and __trunctfxf2 for x86_64 (#66918)

This patch implements __extendxftf2 (long double -> f128) and
__trunctfxf2 (f128 -> long double) on x86_64.
This is a preparation to unblock https://reviews.llvm.org/D53608,
We intentionally do not modify compiler-rt/lib/builtins/fp_lib.h in this
PR
(in particular, to limit the scope and avoid exposing other functions on
X86_64 in this PR).
Instead, TODOs were added to use fp_lib.h once it is available.

Test plan:
1. ninja check-compiler-rt (verified on X86_64 and on Aarch64)
In particular, new tests (extendxftf2_test.c and trunctfxf2_test.c) were
added.
2. compared the results of conversions with what other compilers (gcc)
produce.