[AMDGPU] Use "hostcall" module flag instead of searching for ockl_hostcall_internal() declaration.

The current way to detect hostcalls by looking for "ockl_hostcall_internal()" function in the module seems to be not reliable enough. The LTO may rename the "ockl_hostcall_internal()" function when an application is compiled with "-fgpu-rdc", and MetadataStreamer pass to fail to detect hostcalls, therefore it does not set the "hidden_hostcall_buffer" kernel argument.
This change adds a new module flag: hostcall that can be used to detect whether GPU functions use host calls for printf.

Differential revision: https://reviews.llvm.org/D110337