debug/macho: don't use narch for seenArches map size
If narch is very large we would allocate a lot of memory for seenArches.
In practice we aren't going to see many different architectures so
don't bother to specify a size for the seenArches map.
No debug/macho test case because the problem can only happen for
invalid data. Let the fuzzer find cases like this.
For #47653
For #52523
Change-Id: I5a3b0e3aa6172ddffd6f44d9ae513c39a00d8764
Reviewed-on: https://go-review.googlesource.com/c/go/+/425114
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