nvptx: Partial support for aliases to aliases.

For the following test (adapted from pr96390.c):

__attribute__((noipa)) int foo () { return 42; }
int bar () __attribute__((alias ("foo")));
int baz () __attribute__((alias ("bar")));

int main ()
{
  int n;
  #pragma omp target map(from:n)
    n = baz ();
  return n;
}

gcc emits following ptx for baz:
.visible .func (.param.u32 %value_out) bar;
.alias bar,foo;
.visible .func (.param.u32 %value_out) baz;
.alias baz,bar;

which is incorrect since PTX requires aliasee to be a defined function.
The patch instead uses cgraph_node::get(name)->ultimate_alias_target,
which generates the following PTX:

.visible .func (.param.u32 %value_out) baz;
.alias baz,foo;

gcc/ChangeLog:
	PR target/104957
	* config/nvptx/nvptx.cc (nvptx_asm_output_def_from_decls): Use
	cgraph_node::get(name)->ultimate_alias_target instead of value.

gcc/testsuite/ChangeLog:
	PR target/104957
	* gcc.target/nvptx/alias-to-alias-1.c: Adjust.

Signed-off-by: Prathamesh Kulkarni <prathameshk@nvidia.com>
Co-authored-by: Thomas Schwinge <tschwinge@baylibre.com>