Improve hermetic toolchain support, add static stdlib support

[keith]

Previously hermetic toolchains for the host didn't work correctly
because the shared runtime rpaths wasn't setup correctly. As part of
trying to fix that I implemented static-stdlib support hoping to enable
that by default. Unfortunately the Swift toolchain doesn't ship a static
XCTest version, so you can't use static-stdlib 100% of the time. This
means toolchains that want to support this feature must provide dynamic
and static runtimes, and we pick between those in the rules.

New behavior:

• The hermetic toolchains provide dynamic_runtime (previously
  runtime) and static_runtime files
• By default binary targets (besides swift_test) default to statically
  linking the swift stdlib. This makes these binaries more portable, but
  also makes them larger
• swift_test targets are always linked dynamically
• When linking dynamically, the shared libraries for all Swift stdlib
  libraries are provided for linking, and bazel automatically adds the
  correct rpaths
• When not using any of these features (aka the non-hermetic system
  toolchain), the behavior is unchanged. We still add a non-hermetic
  rpath to the system Swift install directory.
• Defaulting to static linking can be controlled with the
  swift.static_stdlib feature. Setting it on non-linux platforms does nothing
• We provide a default sysroot for ubuntu for static linking, as well as a
  default cc_toolchain based on Swift's installed clang.

General setup:

# MODULE.bazel 
swift = use_extension("//swift:extensions.bzl", "swift", dev_dependency = True)
swift.toolchain(
    name = "swift_toolchain",
    swift_version = "6.3.2",
)
use_repo(swift, "swift_toolchain")

register_toolchains(
    "@swift_toolchain//:cc_toolchain_exec_ubuntu22.04",
    "@swift_toolchain//:swift_toolchain_exec_ubuntu22.04",
    dev_dependency = True,
)

This registers the relevant toolchains for Swift and C++
(required for linking even if you don't have any C/C++)

Closes #1686
Work towards #8
Work towards #1813
Work towards #1816