[opus] Patch cleaning up Opus x86 intrinsics configury

[Jonathan Lennox]

The attached patch cleans up Opus's x86 intrinsics configury.

It:
* Makes —enable-intrinsics work with clang and other non-GCC compilers
* Enables RTCD for the floating-point-mode SSE code in Celt.
* Disables use of RTCD in cases where the compiler targets an instruction set by default.
* Enables the SSE4.1 Silk optimizations that apply to the common parts of Silk when Opus is built in floating-point mode, not just in fixed-point mode.
* Enables the SSE intrinsics (with RTCD when appropriate) in the Win32 build.
* Fixes a case where GCC would compile SSE2 code as SSE4.1, causing a crash on non-SSE4.1 CPUs.
* Allows configuration with compilers with non-GCC-flavor flags for enabling architecture options.
* Hopefully makes the configuration and ifdef’s easier to follow and understand.

This does not yet switch —enable-intrinsics to be enabled by default on supported architectures, but I think it’d be ready to do so.

Comments are welcome!



—
Jonathan Lennox
jonathan at vidyo.com

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