mkbootfs: Link libc++ statically

mkbootfs is in dist_files so it should run
as a standalone binary, but it has a dependency
on libc++.so. Link the C++ library statically
so it can run on hosts that lack that library.

Bug: 409454917
Test: ldd ./out/host/linux-x86/bin/mkbootfs
Change-Id: Id31d5f4aaa0fde9301e6307966c719944960b803