RuntimeLibcalls: Use a single LibcallImpl for long double functions
Replace the per-format <func>l_{f80|f128|ppcf128} implementations with
a single <func>l implementaton. The concrete type is now derived from
the module/triple's LongDoubleFormat, so this is no longer a static
property.
On targets where long double is double, the l-suffixed functions do
not provide a libcall. This could be fixed in principle, but nothing
would use it in the compiler.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>