Drop -ffast-math from Unix REL_FLAGS

PR microsoft#1416 inadvertently regressed PR microsoft#1415 by removing
-fno-finite-math-only from the three Unix REL_FLAGS branches and
silencing the resulting Clang warning via -Wno-nan-infinity-disabled.
That left -ffast-math implying -ffinite-math-only on Clang/AppleClang,
which folds std::isnan / std::isinf to false in nlohmann::json
(lib/include/mat/json.hpp) and breaks the IEEE 754 invariants that
SQLite relies on for sqlite3_bind_double / sqlite3IsNaN.

Rather than restore -fno-finite-math-only, address the root cause by
removing -ffast-math entirely from all three Unix REL_FLAGS branches
(GCC, AppleClang, generic Clang). For a telemetry library:

- The hot paths are string concatenation, Bond/JSON serialization,
  HTTP I/O, and SQLite reads/writes - there is essentially no
  floating-point arithmetic for -ffast-math to optimize.
- Defining __FAST_MATH__ causes some libc++ / glibc headers to
  shortcut isnan / isinf even with -fno-finite-math-only set,
  silently corrupting NaN handling in nlohmann::json output.
- On x86 Linux, -ffast-math links crtfastmath.o which sets MXCSR
  FTZ/DAZ bits process-wide at startup, leaking flush-to-zero
  behavior into client applications that link this SDK - a contract
  violation for any consumer doing scientific compute or audio DSP.
- SQLite source explicitly recommends against -ffast-math.

This subsumes the partial mitigation from microsoft#1415 (-fno-finite-math-only
is no longer needed once -ffast-math itself is gone) and aligns with
the intent of the long-standing draft PR microsoft#1392 by ThomsonTan, extended
to the GCC branch as well (GCC's -ffast-math has the same
finite-math-only and crtfastmath.o behavior on x86 Linux).

Also drop -Wno-nan-infinity-disabled from the Clang WARN_FLAGS - it
only existed to silence the warning that flagged this exact bug, and
is unnecessary once -ffast-math is no longer pulling in
-ffinite-math-only.

Validation:
- cmake configure + Release build of UnitTests on macOS arm64
  (AppleClang 21): no warnings, no -Wnan-infinity-disabled hits.
- ./out/tests/unittests/UnitTests: 486/486 pass.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bmehta001

CMakeLists.txt

@@ -121,20 +121,20 @@ elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
   set(WARN_FLAGS "-Wall -Werror -Wextra -Wno-unused-parameter -Wno-unused-but-set-variable")
 else()
   # No -pedantic -Wno-extra-semi -Wno-gnu-zero-variadic-macro-arguments
-  set(WARN_FLAGS "-Wall -Werror -Wextra -Wno-unused-parameter -Wno-unknown-warning-option -Wno-unused-but-set-variable -Wno-nan-infinity-disabled")
+  set(WARN_FLAGS "-Wall -Werror -Wextra -Wno-unused-parameter -Wno-unknown-warning-option -Wno-unused-but-set-variable")
 endif()
 
 if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
   # Using GCC with -s and -Wl linker flags
-  set(REL_FLAGS "-s -Wl,--gc-sections -Os ${WARN_FLAGS} -ffunction-sections -fdata-sections -fmerge-all-constants -ffast-math")
+  set(REL_FLAGS "-s -Wl,--gc-sections -Os ${WARN_FLAGS} -ffunction-sections -fdata-sections -fmerge-all-constants")
 elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
   set(REL_FLAGS "${WARN_FLAGS}")
 elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "AppleClang")
   # AppleClang does not support -ffunction-sections and -fdata-sections with the -fembed-bitcode and -fembed-bitcode-marker
-  set(REL_FLAGS "-Os ${WARN_FLAGS} -fmerge-all-constants -ffast-math")
+  set(REL_FLAGS "-Os ${WARN_FLAGS} -fmerge-all-constants")
 else()
   # Using clang - strip unsupported GCC options
-  set(REL_FLAGS "-Os ${WARN_FLAGS} -ffunction-sections -fmerge-all-constants -ffast-math")
+  set(REL_FLAGS "-Os ${WARN_FLAGS} -ffunction-sections -fmerge-all-constants")
 endif()
 
 ## Uncomment this to reduce the volume of note warnings on RPi4 w/gcc-8 Ref. https://gcc.gnu.org/ml/gcc/2017-05/msg00073.html