fix: force the .elf suffix on output

To mirror as closely as possible the behavior of Arduino tools.

Author: massonal

Diff for: cmake/toolchain.cmake

@@ -51,3 +51,9 @@ set(BUILD_SHARED_LIBS false CACHE STRING "")
 
 set(CMAKE_CXX_STANDARD 14)
 set(CMAKE_C_STANDARD 11)
+
+set(CMAKE_EXECUTABLE_SUFFIX .elf)
+# These override CMAKE_EXECUTABLE_SUFFIX -- prevent any CMake built-in from overriding the value we want
+set(CMAKE_EXECUTABLE_SUFFIX_C .elf)
+set(CMAKE_EXECUTABLE_SUFFIX_CXX .elf)
+set(CMAKE_EXECUTABLE_SUFFIX_ASM .elf)