[libc] Provide config options for stack protector modes

Currently libc builds can be configured to use or disable strong stack protector. We should be able to configure the build to compile libc with any or none of stack protector modes.

The issue names no files or tests. Start by locating libc's existing build configuration for strong and disabled stack-protector settings, then trace how those options are exposed. Done means the build can select any supported stack-protector mode, including none, with each choice verified.

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