Don't use EBCDIC macro for a C language escape

Don't use EBCDIC macro for a C language escape

C recognizes '\a' (for BEL); just use that instead of a look-up.

regen/unicode_constants.pl could be used to generate the character for
the ESC (set in nearby code), but I didn't do that because of
potential bootstrapping problems when porting to an EBCDIC platform
without a working perl.  (The other characters generated in that .pl are
less likely to cause problems when compiling perl.)