meson: Convert undefsym.sh to undefsym.py

Shell scripts are not easily invoked from the build process
on MSYS, so convert undefsym.sh to a python script.

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Message-Id: <20200902170054.810-3-luoyonggang@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>