[libc] [llvm] [libc][math][c23] Add {,u}fromfp{,x}{,f,l,f128} functions (PR #86003)

[via llvm-commits]

================
@@ -81,3 +81,11 @@ The C standard does not specify behavior for ``printf("%s", NULL)``. We will
 print the string literal ``(null)`` unless using the 
 ``LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS`` option described in :ref:`printf 
 behavior<printf_behavior>`.
+
+Unknown Math Rounding Direction
+-------------------------------
+The C standard states that if the value of the ``rnd`` argument of the
----------------
lntue wrote:

nit: `C23`

https://github.com/llvm/llvm-project/pull/86003