bjam-native: don't do debug builds

bjam-native: don't do debug builds

Previously this recipe was changed to do debug builds because otherwise insane
warns that the binary is already stripped.

However, debug builds for boost.build also pass -O0.  It turns out that given
how large Boost is (or, how bad boost.build is) doing a release build with -O3
knocks a third off the walltime for a Boost package in my test, mainly by reducing
how long it spends deciding that nothing needs to be rebuilt in do_install:

  PKG      TASK        ABSDIFF  RELDIFF  WALLTIME1 -> WALLTIME2
  boost    do_install  -330.7s   -69.2%     477.6s -> 146.9s
  boost    do_compile    -7.1s    -2.7%     269.3s -> 262.2s

Replace debug mode with INSANE_SKIP=already-stripped.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>