statically link adb and fastboot against libc++

libc++.so is not widely available on the host, so we compile against
one built as part of the platform.  This causes problems for adb and
fastboot, which are distributed through a number of channels - the
sdk, distro packages, downloaded from the build server, or manually
copied.  Instead of forcing all users to handle libc++.so too,
statically link against libc++.

Change-Id: I51b75258653a23558c8b598802005f6c1166a439