build: Only probe for faster linker in dev build mode and on x86_64

build: Only probe for faster linker in dev build mode and on x86_64

Using Gold or LLD on less common platforms such as MIPS may not be a
good idea (see e.g.
<https://www.sourceware.org/bugzilla/show_bug.cgi?id=22838>), so be
conservative and only probe for a faster linker on platforms that likely
don’t have toolchain bugs. Also, using a faster linker is in practice
only relevant for dev builds.

Fixes #907.