[Support] Fix build for Haiku

This change fixes two issues with building LLVM on Haiku. The first issue is
that LLVM requires wait4(), which on Haiku is hidden behind the _BSD_SOURCE
feature flag when using the --std=c++14 flag. Additionally, the wait4()
function is only available in libbsd.so, so this is now a dependency.

The other fix is that Haiku does not have the (non-standard) rusage.maxrss
member, so by default the used memory info will be set to 0 on this platform.

Reviewed By: sepavloff

Differential Revision: https://reviews.llvm.org/D87920

Patch by Niels Sascha Reedijk.