ARROW-16757: [C++][FOLLOWUP] Fix mingw32 RTools 4.0 build by removing usage of alignas

[wesm]

Using alignas(64) (instead of alignas(8)) seemed to break this build.

[github-actions]

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[wesm]

I added some checks to prevent the filter implementations from running in the 0-length chunked array cases, and a different filter test started failing:

test_that("[ accepts Arrays and otherwise handles bad input", {
vec<-11:20a<-Array$create(vec)
ind<- c(9, 3, 5)
expect_error(
a[Array$create(ind)],
"Cannot extract rows with an Array of type double"
)
expect_as_vector(a[Array$create(ind-1, type= int8())], vec[ind])
expect_as_vector(a[Array$create(ind-1, type= uint8())], vec[ind])
expect_as_vector(a[ChunkedArray$create(8, 2, 4, type= uint8())], vec[ind])
filt<- seq_along(vec) %in%ind
expect_as_vector(a[Array$create(filt)], vec[filt])
expect_error(
a["string"],
"Cannot extract rows with an object of class character"
)
})

It segfaults on expect_as_vector(a[Array$create(filt)], vec[filt])

This is good because it's a normal array being filtered, but I'm still perplexed why #13521 would have caused this.

[wesm]

@nealrichardson@wjones127@pitrou@paleolimbot so bizarrely, I had mistakenly used alignas(64) instead of alignas(8) with a struct member I added in #13521 -- I don't think that 64-byte alignment for that buffer should necessarily cause something to go wrong, but perhaps this mingw32 in question has a broken implementation of the C++11 alignas keyword? Really strange. In any case I'll clean this up and merge it so builds will stop failing

[github-actions]

https://issues.apache.org/jira/browse/ARROW-16757