Separate backend specific code in redzone_allocator
Separate backend specific code in redzone_allocator

- Moves `LoadKernelOrGetPtr` from `asm_compiler.h` into `redzone_allocator_kernel_cuda.cc` since it's the only user of this function.
- Separate CUDA / ROCm code into separate files: `redzone_allocator_kernel_{cuda|rocm}.cc`.
- Removes all preprocessor branches from `redzone_allocator.cc`.
- Removes unnecessary build macros from `redzone_allocator_test` target definition.
- Reenables ASAN for the `redzone_allocator_test`.
