* Do not attempt to merge non-fat frameworks in Xcode build

During the Xcode build, we strip code irrelevant to the target
architecture in frameworks used by the application. In the case of
non-fat executables, no stripping occurs, so the frameworks can be used
as-is. No merge & replace step is necessary.

* fixup! Do not attempt to merge non-fat frameworks in Xcode build

Required to handle Flutter SDK (and other) paths that include spaces.

Also includes general cleanup:
* Declare explicitly that we use /bin/bash, since we rely on its
  features.
* Add -- where possible, to avoid interpreting files starting in - as
  options.
* Suppress output of pushd/popd.
* Avoid stringifying arrays.

Support for thinning app frameworks to the target architecture was added
in 708909fc. This commit adds support
and error-checking for non-fat frameworks that are not of the target
architecture. In such cases, we now fail the build, and emit an error
message and the contents of lipo -info for the affected framework.

* Support thinning iOS frameworks to supported architectures

When building against frameworks that are distributed as
multi-architecture fat binaries, we want to strip the frameworks we
distribute down to only the architectures specified in $ARCHS.

This patch adds:
* The ability to specify commands to xcode_backend.sh (if none is
  specified, run BuildApp for backward compatibility).
* A 'thin' command that invokes lipo to thin down the distributed as
  described above.

* Add framework thinning step to iOS build

Invokes xcode_backend.sh thin on the build application.

* Limit architectures to arm64 in Xcode template

Flutter does not yet support armv7 iOS devices. Limit the $ARCHS build
variable to arm64 until then.

This change adds a top-level getBuildDirectory func and funcs for
android, aot, asset, ios build products.

Developers may now add a "build-dir" mapping to their
~/.flutter_settings (JSON format) config file. Output directory is
relative to the main flutter application directory.

This change also changes the default build directory for iOS builds to a
subdirectory of the configured build directory, 'build/ios' by default.

Previously, `flutter build` products were emitted to build/ and only
xcode build products were emitted to $SYMROOT.

The `flutter.buildMode` property now controls whether to build in debug,
profiling, or release.

This patch sets a default path for Flutter.framework that's sensible based on
the FLUTTER_MODE.

Add a FLUTTER_MODE variable to the generated Xcconfig and fix snapshot generation on all targets. (#4362)

Get rid of the DART_EXPERIMENTAL_INTERPRETER_FLAG.

This patch adds some sensible defaults to xcode_backend.sh and documents how to
build hello_services for iOS using Xcode.

Fixes #4298

Instead of embedding Flutter.framework and BuildFlutterApp in the Xcode
project, we store these in flutter.git, where we can reference them.