GPU: define CAMath::Abs for GPUdoubleCalcImpl

CAMath::Abs is a template that deduces its parameter rather than taking a float,
so a call on a GPUdoubleCalc intermediate selects the primary template, which is
declared and never defined. Four call sites in the track propagation do exactly
that. Compiling to an object hides it, so the Metal build only fails when the
kernels are linked; on the host the type is a double and the question does not
arise.

Every other CAMath entry point takes a float and is reached through the implicit
conversion, so only Abs needs the specialisation.

Author: ktf

GPU/Common/GPUCommonDouble.h

@@ -16,6 +16,7 @@
 #define GPUCOMMONDOUBLE_H
 
 #include "GPUCommonDef.h"
+#include "GPUCommonMath.h"
 
 #ifndef GPUCA_GPUCODE_DEVICE
 #include <cstdint>
@@ -193,6 +194,15 @@ GPUdi() threadgroup float& operator+=(threadgroup float& a, GPUdoubleCalcImpl b)
 GPUdi() float& operator+=(float& a, GPUdoubleCalcImpl b) { return a = (float)(GPUdoubleCalcImpl(a) + b); }
 #endif
 
+// CAMath::Abs is a template that deduces its parameter, so a call on this type
+// picks the primary template, which has no definition. The rest of CAMath takes
+// float and is reached through the implicit conversion.
+template <>
+GPUhdi() GPUdoubleCalcImpl GPUCommonMath::Abs<GPUdoubleCalcImpl>(GPUdoubleCalcImpl x)
+{
+  return (float)x < 0.f ? -x : x;
+}
+
 // GPUCA_FORCE_DOUBLECALC lets a host test exercise the Metal representation and
 // compare it against the double one.
 #if defined(__METAL__) && defined(__FAST_MATH__)