nvptx: Provide stub 'getpid', 'kill', 'stat'

... as implemented for GCN in 'newlib/libc/sys/amdgcn/*' files, but (for now)
still adding to the catch-all 'newlib/libc/machine/nvptx/misc.c' file.

This is necessary for the GCC/Fortran I/O system, for example.

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

newlib/libc/machine/nvptx/misc.c

@@ -36,11 +36,22 @@ gettimeofday (struct timeval *tv, void *tz) {
   return -1;
 }
 
+int
+getpid (void) {
+  return 0;
+}
+
 int
 isatty (int fd) {
   return fd == 1;
 }
 
+int
+kill (int pid, int sig) {
+  errno = ESRCH;
+  return -1;
+}
+
 off_t
 lseek(int fd, off_t offset, int whence) {
   return 0;
@@ -56,6 +67,12 @@ read(int fd, void *buf, size_t count) {
   return 0;
 }
 
+int
+stat (const char *file, struct stat *pstat) {
+  errno = EACCES;
+  return -1;
+}
+
 void
 sync (void) {
 }