[PATCHv4, MIPS] Add support for O32 FPXX and program header based ABI information

[Matthew Fortune]

> I don't think that it's relevant whether the compiler is native or
> cross.

Indeed. The distinction I was really thinking about was a vendor specific
configuration vs generic 'unknown' configurations but in hindsight this is
not something that needs mentioning here. Updated version below:

+  GCC 5 with binutils 2.25 onwards:
+  It is strongly recommended that all o32 system libraries are built
+  using the new o32 FPXX ABI (-mfpxx) to facilitate the transition as
+  this is compatible with the original and all new o32 ABI
+  extensions.  Configure a MIPS GCC compiler using --with-fp-32=xx to
+  set this by default.

I'll commit it tomorrow unless there are any other comments from anyone.

Thanks,
Matthew