nvptx: Remove newlib ELIX level 1 restriction

nvptx: Remove newlib ELIX level 1 restriction

Such a hard-coded ELIX level restriction is only being applied for nvptx
newlib -- but we'd actually like higher levels' functions available there,
too.  (Users continue to be able to override this via newlib 'configure',
as for every other newlib target.)

This already enables GCC test cases that currently FAIL due to
'unresolved symbol strndup' ('gcc.dg/builtin-dynamic-object-size-0.c'), or
'unresolved symbol mempcpy' ('gcc.dg/torture/pr45636.c'), for example.

Co-authored-by: Andrew Stubbs <ams@codesourcery.com>