Bug#996829: llvm-toolchain-12: 1:12.0.1-9 FTBFS on armel: llvm-ar.cpp.o: undefined reference to symbol '__atomic_fetch_sub_4@@LIBATOMIC_1.0'

[Simon McVittie]

Source: llvm-toolchain-12
Version: 1:12.0.1-9
Severity: serious
Tags: ftbfs
Justification: fails to build from source (but built successfully in the past)

https://buildd.debian.org/status/fetch.php?pkg=llvm-toolchain-12&arch=armel&ver=1%3A12.0.1-9&stamp=1632047469&raw=0
> FAILED: bin/llvm-ar 
> : && /<<PKGBUILDDIR>>/build-llvm/./bin/clang++ -fPIC -Wno-unused-command-line-argument -Wno-unknown-warning-option -fPIC -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wcovered-switch-default -Wno-class-memaccess -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -fdiagnostics-color -ffunction-sections -fdata-sections -O2 -DNDEBUG -g1 -marm -Wl,-z,relro    -Wl,-rpath-link,/<<PKGBUILDDIR>>/build-llvm/tools/clang/stage2-bins/./lib  -Wl,-O3 -Wl,--gc-sections tools/llvm-ar/CMakeFiles/llvm-ar.dir/llvm-ar.cpp.o -o bin/llvm-ar  -Wl,-rpath,"\$ORIGIN/../lib"  lib/libLLVM-12.so.1  -lpthread && :
> /usr/bin/arm-linux-gnueabi-ld: tools/llvm-ar/CMakeFiles/llvm-ar.dir/llvm-ar.cpp.o: undefined reference to symbol '__atomic_fetch_sub_4@@LIBATOMIC_1.0'
> /usr/bin/arm-linux-gnueabi-ld: /usr/lib/arm-linux-gnueabi/libatomic.so.1: error adding symbols: DSO missing from command line

This looks similar to #995827 in llvm-toolchain-13. I can't tell whether
1:12.0.1-10 still has this bug, because it failed in a different way on
armel and armhf (I've opened a separate bug for that).

    smcv