Disable fp-contract on arm64

[andrjohns]

Summary

Resolves floating-point precision errors on arm64 seen in #2935 by disabling the contraction of expressions via the -ffp-contract=off flag

Tests

N/A

Side Effects

Increased precision on arm64

Release notes

Fixed floating-point precision loss on arm64 in some expressions

Checklist

• Math issue Floating point precision loss under arm64 (FP_CONTRACT) #2935

• Copyright holder: Andrew Johnson

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - 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

[andrjohns]

This will be obsoleted by #2937, as part of broader compatibility checks/fixes (sorry for the testing noise!)