nvptx: Disable effective-target 'freestanding'

After 2014's commit 157e859ffe3b5d43db1e19475711c1a3d21ab57a "remove picochip",
the effective-target 'freestanding' (later) was only ever used for nvptx.
However, the relevant I/O library functions have long been implemented in nvptx
newlib.

These test cases generally PASS, just a few need to get XFAILed; see
<https://docs.nvidia.com/cuda/ptx-writers-guide-to-interoperability/#system-calls>,
and then supposedly
<https://docs.nvidia.com/cuda/cuda-c-programming-guide/#formatted-output> for
description of the non-standard PTX 'vprintf' return value:

> Unlike the C-standard 'printf()', which returns the number of characters
> printed, CUDA's 'printf()' returns the number of arguments parsed. If no
> arguments follow the format string, 0 is returned. If the format string is
> NULL, -1 is returned. If an internal error occurs, -2 is returned.

(I've tried a few variants to confirm that PTX 'vprintf' -- which supposedly is
underlying the CUDA 'printf' -- is what's implementing this behavior.)
Probably, we ought to fix that up in nvptx newlib.

	gcc/testsuite/
	* gcc.c-torture/execute/printf-1.c: XFAIL for nvptx.
	* gcc.c-torture/execute/printf-chk-1.c: Likewise.
	* gcc.c-torture/execute/vprintf-1.c: Likewise.
	* gcc.c-torture/execute/vprintf-chk-1.c: Likewise.
	* lib/target-supports.exp (check_effective_target_freestanding):
	Disable for nvptx.

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