[OpenMP] Lower printf to __llvm_omp_vprintf

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.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D112680