[lld-macho] Make everything PIE by default

Modern versions of macOS (>= 10.7) and in general all modern Mach-O
target archs want PIEs by default. ld64 defaults to PIE for iOS >= 4.3,
as well as for all versions of watchOS and simulators. Basically all the
platforms LLD is likely to target want PIE. So instead of cluttering LLD's
code with legacy version checks, I think it's simpler to just default to
PIE for everything.

Note that `-no_pie` still works, so users can still opt out of it.

Reviewed By: #lld-macho, thakis, MaskRay

Differential Revision: https://reviews.llvm.org/D101513