Merge of ea400129c without fef8bd41f into zfs-2.1-release broke i386 build

[mmatuska]

System information

FreeBSD stable/13 i386

Describe the problem you're observing

The changes in ea40012 (More aggsum optimizations) expect the commit fef8bd4 (libspl: implement atomics in terms of atomics) to be landed.

Merge of ea40012 without fef8bd4 broke the build on i386
The code does not build on i386.

Suggested solutions:
Option 1) merge fef8bd4 into zfs-2.1-release
Option 2) add patch to asm-i386/atomic.S from Alexander Motin: https://people.freebsd.org/~mav/zzz3S.patch

Describe how to reproduce the problem

Build OpenZFS 2.1.0-rc7 on i386

Include any warning/errors/backtraces from the system logs

ld: error: undefined symbol: atomic_load_64
>>> referenced by aggsum.c:119 (../../module/zfs/aggsum.c:119)
>>>               .libs/aggsum.o:(aggsum_lower_bound)
>>> referenced by aggsum.c:125 (../../module/zfs/aggsum.c:125)
>>>               .libs/aggsum.o:(aggsum_upper_bound)
>>> referenced by aggsum.c:218 (../../module/zfs/aggsum.c:218)
>>>               .libs/aggsum.o:(aggsum_compare)
>>> referenced 1 more times

ld: error: undefined symbol: atomic_store_64
>>> referenced by aggsum.c:157 (../../module/zfs/aggsum.c:157)
>>>               .libs/aggsum.o:(aggsum_value)
>>> referenced by aggsum.c:158 (../../module/zfs/aggsum.c:158)
>>>               .libs/aggsum.o:(aggsum_value)
>>> referenced by aggsum.c:199 (../../module/zfs/aggsum.c:199)
>>>               .libs/aggsum.o:(aggsum_add)
>>> referenced 3 more times
cc: error: linker command failed with exit code 1 (use -v to see invocation)

[mmatuska]

@behlendorf please take a look into this

[behlendorf]

I've gone ahead and applied the alternate fix of applying all of fef8bd4 to the 2.1 release branch.