[nvptx, libgcc] Fix Wbuiltin-declaration-mismatch in atomic.c

[nvptx, libgcc] Fix Wbuiltin-declaration-mismatch in atomic.c

When building for target nvptx, we get this and similar warnings for libgcc:
...
src/libgcc/config/nvptx/atomic.c:39:1: warning: conflicting types for \
  built-in function ‘__sync_val_compare_and_swap_1’; expected \
  ‘unsigned char(volatile void *, unsigned char,  unsigned char)’ \
  [-Wbuiltin-declaration-mismatch]
...

Fix this by making sure in atomic.c that the pointers used are of type
'volatile void *'.

Tested by rebuilding atomic.c.

libgcc/ChangeLog:

	* config/nvptx/atomic.c (__SYNC_SUBWORD_COMPARE_AND_SWAP): Fix
	Wbuiltin-declaration-mismatch.