[nvptx] Use TARGET_SET_CURRENT_FUNCTION

[nvptx] Use TARGET_SET_CURRENT_FUNCTION

Implement TARGET_SET_CURRENT_FUNCTION for nvptx.  This gives us a place to
add initialization or reset actions that need to be executed on a per-function
basis.

Build and reg-tested on x86_64 with nvptx accelerator.

2018-12-19  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.c (nvptx_previous_fndecl): Declare.
	(nvptx_set_current_function): New function.
	(TARGET_SET_CURRENT_FUNCTION): Define.

From-SVN: r267256