Extension of D112504. Lower amdgpu printf to __llvm_omp_vprintf
which takes the same const char*, void* arguments as cuda vprintf and also
passes the size of the void* alloca which will be needed by a non-stub
implementation of __llvm_omp_vprintf for amdgpu.
This removes the amdgpu link error on any printf in a target region in favour
of silently compiling code that doesn't print anything to stdout.