Log Message:
mozjs78: fix builds on some 32-bit architectures

As with earlier versions, this expects 64-bit atomic operations.
Separately, there's an issue with an assertion in test code that makes
an assumption about the size of an index variable vs. size_t that
doesn't hold.

Test suite comparisons between resultant amd64 and i386 builds on
recent NetBSD releases reveal a similar number of failures (18 vs. 20),
mostly to do with date parsing, though i386 also fails with two IEEE
754 double precision tests.

I did look at carrying mozjs60 patches for NetBSD/powerpc forward, but
I don't have a viable test environment at the moment, and code
inspection suggests that may not be straightforward.