Expose a hosting interface for the JIT.

Expose a hosting interface for the JIT.

This is the first significant step towards removing the JIT's dependence on utilcode.
This change introduces a new interface, `ICorJitHost`, that allows functionality that
would usually be provided by the OS to be overridden by the program that is hosting
the JIT. At the moment, this is limited to memory allocation and configuration value
(e.g. environment variable) access. If the JIT exports a function named `jitStartup`,
an instance of the `ICorJitHost` must be provided via that function before the first
call to `getJit`.

The VM and other implementors of the JIT interface have been updated accordingly.
Most implementors should use the common implementation of `ICorJitHost` (cleverly
named `JitHost`) in utilcode which respects the CLR's hosting policy.

Note that this change does not update RyuJIT (or any other JITs) to use any of the
functionality exposed by `ICorJitHost`; that work is left for future changes.

[tfs-changeset: 1578176]

Commit migrated from https://github.com/dotnet/coreclr/commit/dfd6ab2348b4b151bb90d3bd7deb6a7304bbafac