[CMAKE] Add an option to statically link libstdc++

Summary:
The -static-libstdc++ linker flag is passed manually through the
environment variable LDFLAGS when running configure configure for
the Linux gitian build.

Test Plan:

cmake -DENABLE_STATIC_LIBSTDCXX=ON -GNinja ..
ninja
readelf -d src/bitcoind | grep libstdc++

Grep should return nothing.

cmake -DENABLE_STATIC_LIBSTDCXX=OFF -GNinja ..
ninja
readelf -d src/bitcoind | grep libstdc++

Grep should return a line containing:

Shared library: [libstdc++.so.x]

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2966