Disable NEON on arm in boost unordered.

[greg7mdp]

Disable NEON on arm in boost unordered because of conflicting declaration for float16_t in softfloat library.

Resolves #517.

[greg7mdp]

Will likely be needed in this PR as well.

[spoonincode]

Seems like we should bring this up globally, like for example,

spring/CMakeLists.txt

Lines 178 to 180 in b58cbbc

	# many tests require handling of signals themselves and even when they don't we'd like for them to generate a core dump, this
	# is effectively --catch_system_errors=no broadly across all tests
	add_compile_definitions(BOOST_TEST_DEFAULTS_TO_CORE_DUMP)

Otherwise it's just not practical for everyone to remember to add that define when they include that file

[ericpassmore]

Note:start
group: STABILITY
category: INTERNALS
summary: Disable NEON due to conflict in softfloat library which created problems on ARM builds.
Note:end