libgomp, nvptx: Add fallback for PTX versions lower than 4.1

libgomp, nvptx: Add fallback for PTX versions lower than 4.1

Avoid using the dynamic_smem_size register if the PTX version does not
support it.

This patch should be included when the 'libgomp, nvptx: low-latency memory
allocator' patch is upstreamed.

2022-06-21  Kwok Cheung Yeung  <kcy@codesourcery.com>

	libgomp/
	* config/nvptx/team.c (gomp_nvptx_main): Initialize shared_pool_size
	to zero.  Do not use dynamic_smem_size register if PTX version lower
	than 4.1.