...mostly so that 16d6b7e835ffdcf9b894e79f933dd52348dedd0c
(which reworks unaligned writes in Builder) and the stuff in
https://github.com/haskell/bytestring/pull/631 can see wider testing.

The less-terrible code for unaligned writes used in Builder on
hosts not known to be ulaigned-friendly also takes less effort
for GHC to compile, resulting in a metric decrease for T21839c
on some platforms.

The metric increase on T21839r is caused by the unrelated commit
750dac33465e7b59100698a330b44de7049a345c.  It perhaps warrants
further analysis and discussion (see #23822) but is not critical.

Metric Decrease:
T21839c
Metric Increase:
T21839r