Export ET_LOG_ENABLED=0 as PUBLIC compile definition on cmake targets

[lucylq]

The executorch and executorch_core cmake targets did not propagate
ET_LOG_ENABLED=0 to consumers via INTERFACE_COMPILE_DEFINITIONS.
This caused inline/template functions from headers (e.g.
MemoryAllocator::allocate, Result::CheckOk) to be instantiated
with logging enabled in consumer TUs, pulling in format strings,
et_pal_emit_log_message, and the full POSIX logging stack (fprintf,
chrono).

Adding ET_LOG_ENABLED=0 as a PUBLIC definition ensures consumers
compile header code with the same logging state as the library.

Reduces stripped size_test by 4,096 bytes and stripped
size_test_all_ops by 8,192 bytes.

[lucylq]

Stack from ghstack (oldest at bottom):

• Mark Kernel and KernelKey constructors constexpr #17909
• Use direct function pointers for et_copy_index and et_view in prim_ops #17908
• -> Export ET_LOG_ENABLED=0 as PUBLIC compile definition on cmake targets #17907

[pytorch-bot]

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