libcpu: Include config.h before standard headers in lexer source

libcpu: Include config.h before standard headers in lexer source

As part of the processing of flex, definitions and headers
are added to output source before any literal text or generated code.

This causes standard headers to come before config.h
unless config.h is included in a %top block instead
as specified in the flex manual, section 5.1 "Format of the Definitions".

The %top block is non-POSIX, so using it reinforces
the requirement of "flex" over a standardized "lex" even more.

      * libcpu/i386_lex.l (%top): add flex %top block
      and move config.h header inclusion to it.

Signed-off-by: Michael Pratt <mcpratt@pm.me>