[nvptx] Fix array dimension in nvptx_assemble_decl_begin

When compiling test-case builtin-object-size-21.c, cc1 emits:
...
  .visible .global .align 1 .u32 xm3_3[-2305843009213693951] =
...
for:
...
struct Ax_m3 { char a[PTRDIFF_MAX - 3], ax[]; };

struct Ax_m3 xm3_3 = { { 0 }, { 1, 2, 3 } };
...

Fix this by:
- changing the printing format for unsigned HOST_WIDE_INT init_frag.remaining
  to HOST_WIDE_INT_PRINT_UNSIGNED
- changing the type of local variable elt_size in nvptx_assemble_decl_begin
  to unsigned HOST_WIDE_INT.
such that we have:
...
  .visible .global .align 1 .u32 xm3_3[2305843009213693952] =
...
where 2305843009213693952 == 0x2000000000000000, so the array is claiming
0x8000000000000000 bytes, which is one more than PTRDIFF_MAX.  This is due
to using .u32 instead of .u8, so strictly speaking we should downgrade to
using .u8 in this case, but that corner-case problem doesn't look urgent
enough to fix in this commit.

Build on nvptx, tested with make check-gcc.

gcc/ChangeLog:

	* config/nvptx/nvptx.c (nvptx_assemble_decl_begin): Make elt_size an
	unsigned HOST_WIDE_INT.  Print init_frag.remaining using
	HOST_WIDE_INT_PRINT_UNSIGNED.