nvptx: In offloading execution, map '_exit' to 'abort' [GCC PR85463]

This is still not properly resolving <https://gcc.gnu.org/PR85463>
'[nvptx] "exit" in offloaded region doesn't terminate process', but is
one step into that direction, and allows for simplifying some GCC code.

newlib/libc/machine/nvptx/_exit.c

@@ -14,6 +14,7 @@
  */
 
 #include <unistd.h>
+#include <stdlib.h>
 
 /* Sadly, PTX doesn't support weak declarations, only weak
    definitions.  Weakly define it here in case we're not using crt0
@@ -26,7 +27,15 @@ void __attribute__((noreturn))
 _exit (int status)
 {
   if (__exitval_ptr)
-    *__exitval_ptr = status;
-  for (;;)
-    asm ("exit;" ::: "memory");
+    {
+      *__exitval_ptr = status;
+      for (;;)
+	asm ("exit;" ::: "memory");
+    }
+  else /* offloading */
+    {
+      /* Map to 'abort'; see <https://gcc.gnu.org/PR85463>
+	 '[nvptx] "exit" in offloaded region doesn't terminate process'.  */
+      abort ();
+    }
 }