[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  <[email protected]>

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

Author: vries

gcc/config/nvptx/nvptx.opt

@@ -37,7 +37,7 @@ msoft-stack
 Target Report Mask(SOFT_STACK)
 Use custom stacks instead of local memory for automatic storage.
 
-msoft-stack-reserve-local
+msoft-stack-reserve-local=
 Target Report Joined RejectNegative UInteger Var(nvptx_softstack_size) Init(128)
 Specify size of .local memory used for stack when the exact amount is not known.