Modify Makefile in cros_boot_mode package to use C++ linker.

Build libcros_boot_mode.so with C++ linker rather than the C
linker, since it contains C++ source.

BUG=chromium-os:38770
TEST=Emerged package with new Makefile.

Change-Id: I0154dffb4b3abdc3e354bc235fd838a1b94a0e15
Reviewed-on: https://gerrit.chromium.org/gerrit/43790
Reviewed-by: David James <davidjames@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Caroline Tice <cmtice@chromium.org>
Tested-by: Caroline Tice <cmtice@chromium.org>
1 file changed