Bug 17786 – Define the FreeStanding subset of betterC

As it stands, `-betterC` is a subset of D under which features requiring druntime are unavailable (and should probably be disallowed). `-betterC` however still assumes that the C runtime / standard library is available and the compiler freely emits references to it (such as to memset). As of several releases, the compiler has a FreeStanding predefined version, which according to the spec (https://dlang.org/spec/version.html#predefined-versions) is defined as: An environment without an operating system (such as Bare-metal targets) We should: * Add a compiler switch, so that code targeting bare-metal can actually use this predefined version, and third party libraries could offer bare-metal friendly implementations of their functionalities. * Teach the compiler to not emit references to the C runtime. * Define in the language specification all the features that are not available.

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