And fix things all the way down to it. Namely:
- remove 'r30' from free registers, it's an .LCTOC1 register for gcc. generated .plt stubs expect it to be initialised.
- fix PicBase computation, which originally forgot to use 'tmp' reg in 'initializePicBase_ppc.fetchPC'
- mark 'ForeighTarget's as implicitly using 'PicBase' register (see comment for details)
- add 64-bit MO_Sub and test on alloclimit3/4 regtests
- fix dynamic label offsets to match with .LCTOC1 offset
Signed-off-by: Sergei Trofimovich <firstname.lastname@example.org>