libcdecl: Avoid duplicating keyword strings w/ gperf.

libcdecl: Avoid duplicating keyword strings w/ gperf.

The string table generated by gperf is quite redundant: since we already
resolved to the parser token, we can easily get the string from the
parser's token name table.

Unfortunately gperf provides no options to customize the string table
generation to the extent needed, so let's continue our theme of post-
processing everything in awk.