Added "dexopt for SMP" flag.

Added "dexopt for SMP" flag.

Currently, dexopt always runs on the target device, which means that
decisions about whether or not to convert certain opcodes to SMP-safe
variants could be made based on #ifdefs.

Since we are planning to run dexopt on the host, we need to be able
to configure SMP-mode independently.  This change adds a global variable
that the dexopt code can check.

There is currently no command-line argument to set this, since it will
be set by "dexopt" rather than "dalvikvm", and the exact method of
launching dexopt from the host build is still mildly TBD.

Bug 2981136.

Change-Id: If20aa6f513896fd8b35394388eaca6214f0449a7