nvptx: Add effective-target 'nvptx_softstack', use for effective-target 'alloca'

nvptx: Add effective-target 'nvptx_softstack', use for effective-target 'alloca'

..., and thereby making the check for effective-target 'alloca' more explicit.
As of commit 5012919d0bd344ac1888e8e531072f0ccbe24d2c (Subversion r242503)
"nvptx backend prerequisites for OpenMP offloading", the check for
effective-target 'alloca' did "use a compile test"; let's make this more
explicit: supported for '-msoft-stack', not supported otherwise.

	gcc/testsuite/
	* lib/target-supports.exp
	(check_effective_target_nvptx_softstack): New.
	(check_effective_target_alloca) [nvptx]: Use it.
	gcc/
	* doc/sourcebuild.texi (Effective-Target Keywords): Document
	'nvptx_softstack'.