Here we fix a few portability issues affecting running the validation tests on OpenBSD. The remaining failures will be dealt with separately.
1. In the `rth` tool, ensure `-z origin` is specified to the linker when
using `$ORIGIN`, as required on this platform.
2. Explicitly set the rpath in the `dsohandle-multi-module` execution
test, since the test uses built dynamic libraries during the test.
3. Erase the environment variable instead of using `env -u`, the latter
of which is not portable.
Split up the commands into multiple RUN lines. Use a temporary to
actually capture the output of multiple invocations to compose them into
a single stream.
Replace the `export` usage with `env` which the lit interpreter is able
to process even on Windows.
This makes HashingRandomization pass on Windows.
Newly internal declarations include Hasher._seed and the integer overloads of Hasher._combine(_:), as well as _SipHash13 and _SipHash24.
Unify the interfaces of these SipHash testing structs with Hasher. Update SipHash test to cover Hasher, too.
Add @usableFromInline to all newly internal stuff. In addition to its normal use, it also enables white box testing; compile tests that need to use these declarations with -disable-access-control.
