Investigate V8_ASSUME_ALIGNED warning on macOS

[lloeki]

Has no apparent functional effect on result: mini_racer tests still pass with about the same performance.

Started when building the newly added gyp target libv8_monolith instead of node.

In file included from ../deps/v8/src/compiler/compilation-dependencies.cc:10:
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In file included from ../deps/v8/src/objects/allocation-site-inl.h:10:
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In file included from ../deps/v8/src/heap/heap-write-barrier-inl.h:15:
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In file included from ../deps/v8/src/objects/compressed-slots-inl.h:10:
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../deps/v8/src/common/ptr-compr-inl.h:43:44: warning: requested alignment must be 536870912 bytes or smaller; maximum alignment assumed [-Wbuiltin-assume-aligned-alignment]
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  isolate_root = reinterpret_cast<Address>(V8_ASSUME_ALIGNED(
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                                           ^~~~~~~~~~~~~~~~~~
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../deps/v8/include/v8config.h:364:3: note: expanded from macro 'V8_ASSUME_ALIGNED'
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  __builtin_assume_aligned((ptr), (alignment))
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  ^                               ~~~~~~~~~~~