configure: Don't hard-code strip tool

For reasons that I don't entirely understand we didn't previously detect
`strip` using autoconf. This naturally broke during cross-compilation.
How did this ever work? I have no idea.

Test Plan: Try cross-compiling

Reviewers: austin, hvr, angerman

Subscribers: rwbarton, thomie, erikd

Differential Revision: https://phabricator.haskell.org/D4008