Set as-instr to use printf instead of echo

as-instr was hardcoded to use /bin/echo with the -e option.
Non-GNU systems like MacOSX would fail and cause compiler errors.
Since BSD echo does not have the -e option (escape backslashes)
prinf was used instead would should be more friendly cross-platform
Specifically with the new OSX toolchain it caused the following
error:
  Error: selected processor does not support ARM mode `smc #0'
This was caused by $(call as-instr,.arch_extension sec,+sec) failing
since it was using echo -e.
See: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/echo.html
  "It is not possible to use echo portably across all POSIX systems
   unless both -n (as the first argument) and escape sequences are
   omitted.
  The printf utility can be used portably to emulate any of the
  traditional behaviors of the echo utility as follows (assuming
  that IFS has its standard value or is unset)
  New applications are encouraged to use printf instead of echo."

Change-Id: I247df76c3d59709a88e95882ed0be4883ff52044