Atm we see:
...
libgcc/config/nvptx/crt0.c:36:1: warning: no previous prototype for \
  ‘__main’ [-Wmissing-prototypes]
...

Fix this by adding the prototype.

Build and reg-tested on nvptx.
Build and reg-tested on x86_64 with nvptx accelerator.

2019-06-18  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/crt0.c (__main): Declare.

From-SVN: r272412