[nvptx] Fix -msoft-stack-reserve-local format

[nvptx] Fix -msoft-stack-reserve-local format

Currently, in order to use the switch -msoft-stack-reserve-local with the
default arg 128, you have to specify '-msoft-stack-reserve-local128'.

Fix the switch format such that you specify '-msoft-stack-reserve-local=128'
instead.

Tested on nvptx.

gcc/ChangeLog:

2020-10-12  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.opt (-msoft-stack-reserve-local): Rename to ...
	(-msoft-stack-reserve-local=): ... this.