Request for ideas on arch-specific musl-gcc specfile additions

[Rich Felker <dalias@aerifal.cx>]

Right now, musl-gcc has problems on at least a few archs:

- powerpc: it does not setup the right secure-plt options for dynamic
  linking.

- mips: gcc's internals on mips expect the -EL and -EB options to work
  for selecting endianness, but they're not real options; they're
  created by the specfile

- any arch with 32/64-bit variants: if -m32 or -m64 was used building
  musl, it should get added to the specfile for musl-gcc.

I'm looking for a clean approach to fixing this. Certainly the
configure script could just generate one or more extra variables that
get stored in config.mak and substituted into the specfile, but I'd
rather avoid having essential logic like this in configure, as it
makes it difficult for anybody who wants to generate config.mak by
hand. If there are specific options always needed on a given arch, the
build process (even without configure being used) should automatically
add them to the specfile.

Any good proposals?

Rich