Get rid of warnings about non-literal printf() format strings on darwin.

On darwin we get warnings from the gen compiler about non-literal
printf() format strings in these three tests, causing them to fail.
Resolve these warnings as follows:

test/extern/hilde/namedExtern.chpl
  We declare printf() such that the fmt argument is implicitly typed
  by the corresponding actual argument.  That actual arg is a literal
  string, but of type Chapel string.  The compiler emits code to
  convert it to a c_string for the call to the extern printf(), but
  this ends up making the actual a variable in the emitted code.
  Change the printf() declaration(s) so that the fmt argument is
  explicitly a c_string.  This makes the compiler emit the literal as
  a c_string in the first place, and we don't get conversion code or
  the resulting variable.

test/io/vass/time-write.chpl
  In this case we actually do need to be able to printf() non-literal
  strings.  As the code stands we send these as the first (format)
  argument to printf().  Since we need to prnt non-literal strings,
  change the calls (and declarations) so that we use a "%s" format and
  pass the c_string to print as the second argument.  Arguably this
  adds a slight amount of overhead to the printf() call and thus
  changes the test.  If this is a problem we'll need to discuss the
  point of the test, in particular whether it is testing printf()
  performance or the performance of the C buffered I/O FILE interface.
  If the latter, we could call puts() here instead of printf().

test/studies/pidigits/bbp/nomem32.chpl
  This has the same problem as namedExtern, above.  The printf("\t")
  here is the only printf() call in the program.  Everywhere else we
  use Chapel write.  Convert the printf() to a write().