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.

arch/risc-v/src/cmake/platform.cmake

@@ -45,7 +45,7 @@ execute_process(
 
 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/tricore/src/cmake/platform.cmake

@@ -46,7 +46,7 @@ if(CONFIG_TRICORE_TOOLCHAIN_GNU)
     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