nvptx: Don't use global constructor for SSP setup

Given that nvptx newlib currently restricts itself to ELIX level 1, this
is not already a problem.  However, in the following we'd like to lift
that restriction, and then run into:

    [...]/newlib/libc/ssp/stack_protector.c: In function ‘__stack_chk_init’:
    [...]/newlib/libc/ssp/stack_protector.c:31:1: sorry, unimplemented: global constructors not supported on this target
       31 | }
          | ^

GCC patch "nvptx: Support global constructors/destructors via 'collect2'"
has been posted, but not yet accepted.  Until that is resolved, use the
same manual SSP setup as for GCN.

newlib/libc/ssp/stack_protector.c

@@ -5,8 +5,8 @@
 #include <string.h>
 #include <unistd.h>
 
-#if defined(__AMDGCN__)
-/* GCN does not support constructors, yet.  */
+#if defined(__AMDGCN__) || defined(__nvptx__)
+/* Global constructors not supported on this target, yet.  */
 uintptr_t __stack_chk_guard = 0x00000aff; /* 0, 0, '\n', 255  */
 
 #else