Remove support for Solaris on PPC and for old versions.

Remove support for Solaris on PPC and for old versions.
* s/sol2-6.h: Add the contents of s/sol-2.3.h, s/sol-2.4.h, s/sol-2.5.h.
(LD_SWITCH_SYSTEM, USE_MMAP_FOR_BUFFERS): Remove #defines/#undef
that cancel each other.
* s/sol2-3.h:
* s/sol2-4.h:
* s/sol2-5.h: Remove.
* m/ibmrs6000.h: Remove code for USG5_4, this file is only used on AIX.
(NO_REMAP): Remove, unused.
(UNEXEC): Move definition ...
* s/aix4-2.h (UNEXEC): ... here.

* configure.in:	Remove support for Solaris on PPC and for old versions.