Add function to get architectures of parent images so each gcc image has the same set

[yosifkit]

No hurry. 😉

$ # master branch
$ ./generate-stackbrew-library.sh > gcc-old
$ # archy branch
$ ./generate-stackbrew-library.sh > gcc-new
$ diff gcc-old gcc-new
1c1
< # this file is generated via https://github.com/docker-library/gcc/blob/87dfadb46c2a905abb1bcd383ec15028e4454435/generate-stackbrew-library.sh
---
> # this file is generated via https://github.com/docker-library/gcc/blob/f71e9f9cec20b66bfbab98fd1dc8df78f86647c8/generate-stackbrew-library.sh
8a9
> Architectures: amd64
14a16
> Architectures: amd64
20a23
> Architectures: amd64
26a30
> Architectures: amd64

See also docker-library/buildpack-deps#59.

[tianon]

Lol, this will be a lot more fun once docker-library/official-images#3041 merges. 😄

[tianon]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77447

I think for now, we should explicitly exclude arm32v* with a link to that bug.

https://sources.debian.net/src/gcc-6/6.3.0-18/debian/rules2/#L491-L536 is another good link (where Debian adds a bunch of extra compile flags to ARM builds, like specifying exactly which ARM we're compiling for, and the features of it, which should fix our current build failure there: /usr/include/features.h:364:12: fatal error: sys/cdefs.h: No such file or directory)

[tianon]

(filed an issue to track arm32v7 by itself: #37)

[tianon]

👍 LGTM