nvptx target: Global constructor, destructor support, via nvptx-tools 'ld'

The function attributes 'constructor', 'destructor', and 'init_priority' now
work, as do the C++ features making use of this.  Test cases with effective
target 'global_constructor' and 'init_priority' now generally work, and
'check-gcc-c++' test results greatly improve; no more
"sorry, unimplemented: global constructors not supported on this target".

For proper execution test results, this depends on
<https://github.com/SourceryTools/nvptx-tools/commit/96f8fc59a757767b9e98157d95c21e9fef22a93b>
"ld: Global constructor/destructor support".

	gcc/
	* config/nvptx/nvptx.h: Configure global constructor, destructor
	support.
	gcc/testsuite/
	* gcc.dg/no_profile_instrument_function-attr-1.c: GCC/nvptx is
	'NO_DOT_IN_LABEL' but not 'NO_DOLLAR_IN_LABEL', so '$' may apper
	in identifiers.
	* lib/target-supports.exp
	(check_effective_target_global_constructor): Enable for nvptx.
	libgcc/
	* config/nvptx/crt0.c (__gbl_ctors): New weak function.
	(__main): Invoke it.
	* config/nvptx/gbl-ctors.c: New.
	* config/nvptx/t-nvptx: Configure global constructor, destructor
	support.