[lldb] Simplify specifying of platform supported architectures

The GetSupportedArchitectureAtIndex pattern forces the use of
complicated patterns in both the implementations of the function and in
the various callers.

This patch creates a new method (GetSupportedArchitectures), which
returns a list (vector) of architectures. The
GetSupportedArchitectureAtIndex is kept in order to enable incremental
rollout. Base Platform class contains implementations of both of these
methods, using the other method as the source of truth. Platforms
without infinite stacks should implement at least one of them.

This patch also ports Linux, FreeBSD and NetBSD platforms to the new
API. A new helper function (CreateArchList) is added to simplify the
common task of creating a list of ArchSpecs with the same OS but
different architectures.

Differential Revision: https://reviews.llvm.org/D113608