Adding support for replacing ljmps with sjmps in jumptables

Adding support for replacing ljmps with sjmps in jumptables
generated for switch statements. For now you need to set the
environment variable SDCC_SJMP_JUMPTABLE to enable this.
Now 4 algorithms for mcs51 jumptable generation are used:
ljmp or sjmp jumptables for up to 16 cases, stack-pushing target
addresses loaded pc-relative for up to 112 cases and stack-pushing
target addresses loaded with offset from dptr for up to 256 cases.

* src/SDCCpeeph.c: added peephole conditional labelJTInRange
* src/mcs51/main.c: adapted constants for switch table generation
* src/mcs51/peeph.def: added 260.x for replacing ljmp with sjmp

git-svn-id: https://sdcc.svn.sourceforge.net/svnroot/sdcc/trunk/sdcc@3590 4a8a32a2-be11-0410-ad9d-d568d2c75423