Big Endian Detection: Add a whitelist of always BE OSs

[avar]

Hopefully fix the issue with AIX falling through this detection logic
and being detected as Little Endian reported on the Git mailing list,
see
https://public-inbox.org/git/20180729200623.GF945730@genre.crustytoothpaste.net/

Attempt to solve this by extending the existing fallback we have for
detecting always Big Endian processors to also detecting always Big
Endian OSs. See the public-inbox link in the comment I'm adding for
why this should work.

I have not tested this myself, since I have no access to AIX. I'll be
pointing Michael Felt to the relevant MR so he can test it before this
is merged.

[avar]

As it turns out I've been able to test this myself on AIX (I got access to the GCC compile farm), and I can confirm that this works. On xlc 13.1.3 we take this branch, but on GCC 7.2.0 we take the earlier GCC-specific codepath.

I submitted this without knowing about #42 (even though I originally said they could submit a PR here, I forgot), looking at that PR I think this is probably a better approach, since I'm told that AIX is always BE, stuff that isn't AIX won't define _AIX, and it's useful to have this "is this OS always BE?" facility for future additions.

@MrShark @jrn @shumow @cr-marcstevens @kencu @aixtools: What do you think?

[aixtools]

I think your process is very gcc dependent and/or very gnu dependent.

root@x066:[/data/prj/aixtools/git/sha1test]make clean
find . -type f -name '.a' -print -delete
find: 0652-017 -delete is not a valid option.
make: [Makefile:102: clean] Error 1 (ignored)
find . -type f -name '.d' -print -delete
find: 0652-017 -delete is not a valid option.
make: [Makefile:103: clean] Error 1 (ignored)
find . -type f -name '.o' -print -delete
find: 0652-017 -delete is not a valid option.
make: [Makefile:104: clean] Error 1 (ignored)
find . -type f -name '.la' -print -delete
find: 0652-017 -delete is not a valid option.
make: [Makefile:105: clean] Error 1 (ignored)
find . -type f -name '.lo' -print -delete
find: 0652-017 -delete is not a valid option.
make: [Makefile:106: clean] Error 1 (ignored)
find . -type f -name '.so' -print -delete
find: 0652-017 -delete is not a valid option.
make: [Makefile:107: clean] Error 1 (ignored)
find . -type d -name '.libs' -print | xargs rm -rv
rm -rf bin

mkdir -p bin && ar cru bin/libsha1detectcoll.a obj_lib/ubc_check.lo obj_lib/sha1.lo
cc obj_src/main.lo -Lbin -lsha1detectcoll -o bin/sha1dcsum
cc: 1501-218 (W) file obj_src/main.lo contains an incorrect file suffix
cc obj_src/main.lo -Lbin -lsha1detectcoll -o bin/sha1dcsum_partialcoll
cc: 1501-218 (W) file obj_src/main.lo contains an incorrect file suffix

Anyway, using xlc it also seems to PASS:

root@x066:[/data/prj/aixtools/git/sha1test]make test
test e98a60b463a6868a6ce351ab0166c0af0c8c4721 != bin/sha1dcsum test/sha1_reducedsha_coll.bin | cut -d' ' -f1 || (echo "\nError: Compiled for incorrect endianness" && false)
test a56374e1cf4c3746499bc7c0acb39498ad2ee185 = bin/sha1dcsum test/sha1_reducedsha_coll.bin | cut -d' ' -f1
test 16e96b70000dd1e7c85b8368ee197754400e58ec = bin/sha1dcsum test/shattered-1.pdf | cut -d' ' -f1
test e1761773e6a35916d99f891b77663e6405313587 = bin/sha1dcsum test/shattered-2.pdf | cut -d' ' -f1
test dd39885a2a5d8f59030b451e00cb45da9f9d3828 = bin/sha1dcsum_partialcoll test/sha1_reducedsha_coll.bin | cut -d' ' -f1
test d3a1d09969c3b57113fd17b23e01dd3de74a99bb = bin/sha1dcsum_partialcoll test/shattered-1.pdf | cut -d' ' -f1
test 92246b0b718f4c704d37bb025717cbc66babf102 = bin/sha1dcsum_partialcoll test/shattered-2.pdf | cut -d' ' -f1
bin/sha1dcsum test/*
a56374e1cf4c3746499bc7c0acb39498ad2ee185 test/sha1_reducedsha_coll.bin
16e96b70000dd1e7c85b8368ee197754400e58ec coll test/shattered-1.pdf
e1761773e6a35916d99f891b77663e6405313587 coll test/shattered-2.pdf
bin/sha1dcsum_partialcoll test/*
dd39885a2a5d8f59030b451e00cb45da9f9d3828 coll test/sha1_reducedsha_coll.bin
d3a1d09969c3b57113fd17b23e01dd3de74a99bb coll test/shattered-1.pdf
92246b0b718f4c704d37bb025717cbc66babf102 coll test/shattered-2.pdf

[avar]

@aixtools I test this on AIX by building git with it, i.e. manually applying this patch to the sha1dc/sha1.c file, but the above also looks good!

[shumow]

Thank you Ævar and Michael.