[libdevice] Mark external C functions weak after f07988ff3ec8
We relies on prepare-builtins.cpp to
`Set linkage of every external definition to linkonce_odr`
With f07988ff3ec8, prepare-builtins is no longer called,
so we need to mark the weak in source directly.
Fixes multiple definition of `__dpcpp_nativecpu_get_global_id'
etc in
SYCL :: native_cpu/multiple_tu.cpp
