[openmp][amdgpu] Make plugin robust to presence of explicit implicit arguments

OpenMP (compiler) does not currently request any implicit kernel
arguments. OpenMP (runtime) allocates and initialises a reasonable guess at
the implicit kernel arguments anyway.

This change makes the plugin check the number of explicit arguments, instead
of all arguments, and puts the pointer to hostcall buffer in both the current
location and at the offset expected when implicit arguments are added to the
metadata by D113538.

This is intended to keep things running while fixing the oversight in the
compiler (in D113538). Once that patch lands, and a following one marks
openmp kernels that use printf such that the backend emits an args element
with the right type (instead of hidden_node), the over-allocation can be
removed and the hardcoded 8*e+3 offset replaced with one read from the
.offset of the corresponding metadata element.

Reviewed By: estewart08

Differential Revision: https://reviews.llvm.org/D114274