Ensure pointer is 8-byte aligned in stack_alloc::alloc

[WardBrian]

Summary

This was originally found using UBSAN on the csr_matrix_times_vector function: stan-dev/rstan#1111

The issue there was that that function places u, which is std::vector<int>, in our memory arena. If u contained an odd number of elements (e.g., 3 in the above issue), this resulted in a call to alloc with a len of 12.

The next call to alloc would then return a pointer which was not 8-byte aligned, which is undefined behavior. On x86 this is relatively benign, but on other platforms this could be either very slow or cause processor exceptions.

Tests

I have updated stack_alloc_test

Side Effects

This will add a few unused bytes (always less than 8) to allocations of types that sizeof(T) % 8 !=0. Additionally, a bit of extra math is needed in the alloc function, but @SteveBronder and I checked that this gets compiled down to just a and, sub, and cmov

Release notes

Fixed the stack_allocator being able to return non-8-byte aligned pointers

Checklist

• Copyright holder: (fill in copyright holder information)

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  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[SteveBronder]

At some point I think we should come back and rewrite some of the memory things here so that we can request arbitrary alignments, but I think good!