CMake: Fix escaping of quotes in response file

We put macros in a response file compile_time_defs.txt and pass it
to the compiler. When contents of the response file get extracted,
a macro like -DFOO=\"BAR\" gets unescaped into -DFOO="BAR" which
sets FOO to BAR without quotes. To fix this, add a pair of single
quotes around each macro, e.g. '-DFOO=\"BAR\"' in the response file
which becomes one-level processed into -DFOO=\"BAR\".

tools/cmake/mbed_toolchain.cmake

@@ -17,7 +17,7 @@ function(mbed_generate_options_for_linker target output_response_file_path)
 
     # Append -D to all macros as we pass these as response file to cxx compiler
     set(_compile_definitions
-        "$<$<BOOL:${_compile_definitions}>:-D$<JOIN:${_compile_definitions}, -D>>"
+        "$<$<BOOL:${_compile_definitions}>:'-D$<JOIN:${_compile_definitions},' '-D>'>"
     )
     file(GENERATE OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/compile_time_defs.txt" CONTENT "${_compile_definitions}\n")
     set(${output_response_file_path} @${CMAKE_CURRENT_BINARY_DIR}/compile_time_defs.txt PARENT_SCOPE)