[libclc] [libclc] Move __clc_ldexp to CLC library (PR #126078)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> Good to know, thanks. No one else in libclc is using `__builtin_ldexp`, so in its current form it'd still have to be some sort of a customization. Everyone is opting for the software implementation.

Really we just want to get to llvm.ldexp. It's unfortunate we're stuck routing through these old GCC builtins that have any libm baggage associated with them

> Would we ideally have an `__builtin_elementwise_ldexp` to target the vector intrinsics directly?

Yes

https://github.com/llvm/llvm-project/pull/126078