gdb: Flexible target matching for darwin
Outside of the nix-build the target is `x86_64-apple-darwin17.4.0`,
while inside the target is `x86_64-apple-darwin`. This difference
causes the fallback target configuration for darwin, which disables
gdb. Add a patch to make the target matching more flexible.
(cherry picked from commit 4c76a21aae4299b3ec980f0f56e2d7f2860a8826)
(cherry picked from commit fe0728fa2cf5b0e971a829d30182f04331d26c18)
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