Build glitches for arm + GCC 5.4

[Michael Plass <mfpnb%plass-family.net@localhost>]

I've run into some small problems trying to build netbsd-current for evbarm
(RPI2 in my case). I've mostly sorted through them, but I thought I'd share
what I needed to do.

I've tried both a cross build using FreeBSD 8.4 and a native build
on NetBSD 7.0 (PRI2).

For both build methods, the dependall step for 
external/gpl3/gcc/usr.bin/backend/arm-builtins.d
failed because a generated header file had not been created. A patch
to fix this is at

  https://gist.github.com/mfplass/1c66d050bf4a678ddb0c21b97f39f449

See also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57683
I'm not using a parallel build, but the serial build seems to hit it
every time. I'm curious why this hasn't been reported before.

For the cross build, I first needed to apply the patch for toolchain-51118 to
get the freebsd build to work (this does not have to do with gcc 5.4).
But the build fails, apparently because the host c++ compiler is not up to
the task: https://gist.github.com/mfplass/202df0d362501a522ca2b257977a6b80
(I suspect a newer version of freebsd would not have this problem).

I was able to get a cross build to work by asking for gcc 4.8 (HAVE_GCC=48).

The native build was failing at first by running out of memory. I needed to add
a swap file because running genautomata takes nearly 1 gig of RAM. There's a gcc
bug report about that as well: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70473
It seems to be working now, but it is still working on compiling all the various
kernels - it does take a long time! I'm waiting for it to finish before I test the
cross-built image.

Thanks,
- Michael