Support interpolated function pointers in @gcsafe_ccall on Julia < 1.12

[maleadt]

@gcsafe_ccall is meant to be a drop-in replacement for @ccall, but it failed on function pointers interpolated with $, at least on Julia versions without native support for GC-safe calls (anything before 1.12). Those versions go through GPUToolbox's own lowering, which passed the $ expression straight to ccall instead of evaluating it. As a result:

julia> fptr = cglobal(:jl_rand);

julia> @gcsafe_ccall $fptr()::UInt64
ERROR: syntax: "$" expression outside quote

On Julia 1.12+ the macro defers to Base's lowering, which handles this fine, so the same code works there and fails on 1.10 and 1.11.

This comes up with APIs that are only reachable through function pointers. OpenCL extension functions are an example: they're looked up with clGetExtensionFunctionAddressForPlatform, and OpenCL.jl wants to call them GC-safely like its other API calls (JuliaGPU/OpenCL.jl#518). For now it has to fall back to Base's @ccall gc_safe=true on 1.12+ and a plain @ccall before that.

This PR makes the fallback handle interpolated pointers the same way Base's @ccall does. It evaluates the expression once, checks that it's a Ptr{Cvoid} (throwing the same ArgumentError as Base's @ccall does on those Julia versions), and calls through it inside the GC-safe region.

The tests now call an interpolated pointer, and on the fallback path also check the error for a non-pointer value. They pass on Julia 1.10 through 1.13 and on nightly.