[nvptx] Fix printing of 128-bit constant (negative case)

[nvptx] Fix printing of 128-bit constant (negative case)

For this code:
...
__int128 min_one = -1;
...
we currently generate:
...
.visible .global .align 8 .u64 min_one[2] = { -1, 0 };
...

Fix this in nvptx_assemble_value, such that we have instead:
...
.visible .global .align 8 .u64 min_one[2] = { -1, -1 };
...

gcc/ChangeLog:

	* config/nvptx/nvptx.c (nvptx_assemble_value): Handle negative
	__int128.

gcc/testsuite/ChangeLog:

	* gcc.target/nvptx/int128.c: New test.