Made bitfield a distinct type from bit so that bitfields

Made bitfield a distinct type from bit so that bitfields
convert as per ANSI C and bits retain their traditional
boolean style behaviour. Implemented bitfield support in
the z80 port.

* src/SDCCsymt.h,
* src/SDCCsymt.c,
* src/SDCCast.c,
* src/cdbFile.c,
* src/mcs51/gen.c,
* src/ds390/gen.c: bit v bitfield split
* src/z80/gen.c: New support for bitfields
* support/regression/tests/bitfields.c: reenabled z80,
added more tests

git-svn-id: https://sdcc.svn.sourceforge.net/svnroot/sdcc/trunk/sdcc@2836 4a8a32a2-be11-0410-ad9d-d568d2c75423