[build] correctly set -fstack-reuse=none only for gcc
[build] correctly set -fstack-reuse=none only for gcc

Summary:
currently this will add the -fstack-reuse=none flag to all builds with
all compilers except those who set ENABLE_CLANG_TIDY=ON even though only
gcc knows about it

this only adds -fstack-reuse=none to gcc builds

Test Plan:
  cmake -GNinja
  ninja -vn | grep fstack-reuse

see correct ninja dry-run with option set for gcc (it is the default compiler on my box)

  cmake -GNinja -DCMAKE_CXX_COMPILER=clang++
  ninja -vn | grep fstack-reuse

see no lines

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7428

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