#1820: Clean up C preprocessor macros

[Chicken Trac]

#1820: Clean up C preprocessor macros
-----------------------------------------+----------------------------
            Reporter:  felix winkelmann  |       Type:  task
              Status:  new               |   Priority:  minor
           Milestone:  5.4               |  Component:  core libraries
             Version:  5.3.0             |   Keywords:
Estimated difficulty:  easy              |
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 We can replace `C_externimport` and `C_externexport` with `C_extern`.
 Also, `C_fcall` can be removed. Possibly also `C_regparm` and `C_ccall` as
 these refer to obsolete features.

 `C_TLS` was never used and should go.

 Note: `C_externimport` and `C_externexport` are produced bz the compiler,
 so need to be retained for at least one release.

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Ticket URL: <https://bugs.call-cc.org/ticket/1820>
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