GCC: different OS targets.

[David Edelsohn]

	The PowerPC port of GCC can be configured for AIX or for SVR4.
The SVR4 port allows an AIX-style calling convention as compatibility for
a very old embedded PowerPC environment that Cygnus once produced.  One
cannot build a single compiler for full support of both ABIs.  Linux,
eABI, Solaris, and SysV are all SVR4 variants and can be selected with a
command-line option to a single compiler.

David