Disable C++ static denstructors in ARMC6 compiler

We disable C++ static destructors as best as possible in the various
toolchains, trying to eliminate unwanted destructor code. We don't want
to waste RAM or ROM on the C++-standard-specified behaviour of running
all statically-constructed objects' destructors in reverse order on
exit; we're never going to perform an orderly exit.

Techniques used include:

* `SingletonPtr` - makes an object be "lazily constructed" on first use,
  and also eliminates its destructor. Lazy construction adds ROM+RAM
  overhead.
* `__eabi_atexit` is stubbed out, preventing RAM usage by run-time
  registration of static destructors.
* GCC has `exit` wrapped to kill shutdown code
* IAR has static destructors disabled in the compiler flags

Killing static destructors in the compiler is the optimum; if we only
stub out `__eabi_atexit`, the compiler is still inserting calls to it,
and referencing the destructors in those call.

Clang 8 added the compiler option `-fno-c++-static-destructors` (and the
object attributes `[[clang::no_destroy]]` and
`[[clang::always_destroy]]` for fine control).

We can hence enable that option in ARMC6 tool profiles, matching IAR.

This option appears to exist in ARM Compiler 6.11, but generates an
apparently spurious linker error about `EthernetInterface`. It works in
ARM Compiler 6.13, so this PR needs to wait until the compiler is
updated.

tools/profiles/debug.json

@@ -24,7 +24,7 @@
                    "-DMBED_DEBUG", "-DMBED_TRAP_ERRORS_ENABLED=1"],
         "asm": [],
         "c": ["-D__ASSERT_MSG", "-std=gnu11"],
-        "cxx": ["-fno-rtti", "-std=gnu++14"],
+        "cxx": ["-fno-rtti", "-fno-c++-static-destructors", "-std=gnu++14"],
         "ld": ["--verbose", "--remove", "--show_full_path", "--legacyalign",
                "--any_contingency", "--keep=os_cb_sections"]
     },

tools/profiles/develop.json

@@ -23,7 +23,7 @@
                    "-DMBED_TRAP_ERRORS_ENABLED=1"],
         "asm": [],
         "c": ["-D__ASSERT_MSG", "-std=gnu11"],
-        "cxx": ["-fno-rtti", "-std=gnu++14"],
+        "cxx": ["-fno-rtti", "-fno-c++-static-destructors", "-std=gnu++14"],
         "ld": ["--show_full_path", "--legacyalign", "--any_contingency",
                "--keep=os_cb_sections"]
     },

tools/profiles/release.json

@@ -23,7 +23,7 @@
                    "-DNDEBUG"],
         "asm": [],
         "c": ["-D__ASSERT_MSG", "-std=gnu11"],
-        "cxx": ["-fno-rtti", "-std=gnu++14"],
+        "cxx": ["-fno-rtti", "-fno-c++-static-destructors", "-std=gnu++14"],
         "ld": ["--show_full_path", "--legacyalign", "--any_contingency",
                "--keep=os_cb_sections"]
     },