Hi,

This patch merges the libdruntime sub-library with upstream druntime 4b2674b3.

Adds version (BacktraceExternal) for using libexecinfo instead of
internal implementation on FreeBSD, NetBSD, and DragonFly.

Bootstrapped and regression tested on x86_64-linux-gnu, with
preliminary build testing done on x86_64-freebsd11.2,
x86_64-dragonfly5.4, and x86_64-netbsd8.0.

Committed to trunk as r270482.

-- 
Iain
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