Make bitcoin-util use the emulator as needed

Summary:
By reading the config file, bitcoin-util can determine if an emulator is
needed. This is useful for running with wine or qemu.

Depends on D5878.

Test Plan:
Prerequisite

sudo apt install qemu-user-static

Then:

cmake -GNinja .. \
  -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/LinuxAArch64.cmake \
  -DCMAKE_CROSSCOMPILING_EMULATOR=$(command -v qemu-aarch64-static)
QEMU_LD_PREFIX=/usr/aarch64-linux-gnu ninja check-bitcoin-util

Sanity check for regression:

cmake -GNinja ..
ninja check-bitcoin-util

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5879