Add support for OpFRem

Add support for OpFRem

This is the `other` remainder from OpFMod, which we already had.
Uses the existing RValue<Float4> % RValue<Float4> operator wiring; the
only plumbing I had to fix was that llvm emits a call to fmodf, which
was made available in ExternalFunctionSymbolResolver.

Test: dEQP-VK.spirv_assembly.instruction.compute.opfrem.all
Test: dEQP-VK.spirv_assembly.instruction.graphics.frem.*
Bug: b/126873455
Change-Id: I81fbfd8f9e5cab6ac2f16319edda031a122e3a4d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27370
Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>