Add CODEPATCH_CODE() macro to simplify defining a symbol for a chunk

Add CODEPATCH_CODE() macro to simplify defining a symbol for a chunk
of code to use in codepatching.  Use that for all the existing
codepatching snippets.

Similarly, add CODEPATCH_CODE_LEN() which is CODEPATCH_CODE() but also
provides a short variable holding the length of the codepatch snippet.
Use that for some snippets that will be used for retpoline replacement.

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