Add a static assertion macro

[maleadt]

Makes it easy to write functions that only work in certain situations:

julia> using GPUCompiler
julia> include("examples/kernel.jl")

julia> kernel() = GPUCompiler.@static_assert false "you need true"

julia> main()
ERROR: InvalidIRError: compiling MethodInstance for kernel() resulted in invalid LLVM IR
Reason: static assertion failed (you need true)
Stacktrace:
 [1] macro expansion
   @ ~/.julia/packages/LLVM/zkwaZ/src/interop/base.jl:39
 [2] macro expansion
   @ ~/Julia/pkg/GPUCompiler/src/static_assert.jl:19
 [3] kernel
   @ ./REPL[5]:1

julia> kernel() = GPUCompiler.@static_assert true "you need true"

julia> main()
works fine

Also, a drive-by breaking change, removing the PTX version emission. I plan on moving that to CUDA.jl and using the static assertion functionality from here, which isn't a problem as we haven't released a CUDA.jl version compatible with GPUCompiler 2.x yet.