libm: Do not link the toolchain's libm unless explicitly selected

Fix CMake-based build system to include the toolchain's libm only
when `CONFIG_LIBM_TOOLCHAIN` is selected. Before this commit, if
the user selected `CONFIG_LIBM_NEWLIB`, for instance, the build
system would still link the toolchain's libm functions instead of
the ones provided by newlib.

PS: this commit applies the same changes previously introduced for
the other architectures.

arch/arm64/src/cmake/platform.cmake

@@ -42,7 +42,7 @@ execute_process(
   OUTPUT_STRIP_TRAILING_WHITESPACE
   OUTPUT_VARIABLE extra_library)
 list(APPEND EXTRA_LIB ${extra_library})
-if(NOT CONFIG_LIBM)
+if(CONFIG_LIBM_TOOLCHAIN)
   execute_process(
     COMMAND ${CMAKE_C_COMPILER} ${CMAKE_C_FLAG_ARGS} ${NUTTX_EXTRA_FLAGS}
             --print-file-name=libm.a

arch/xtensa/src/cmake/platform.cmake

@@ -42,7 +42,7 @@ execute_process(
   OUTPUT_STRIP_TRAILING_WHITESPACE
   OUTPUT_VARIABLE extra_library)
 list(APPEND EXTRA_LIB ${extra_library})
-if(NOT CONFIG_LIBM)
+if(CONFIG_LIBM_TOOLCHAIN)
   execute_process(
     COMMAND ${CMAKE_C_COMPILER} ${CMAKE_C_FLAG_ARGS} ${NUTTX_EXTRA_FLAGS}
             --print-file-name=libm.a