IAR: Improve support for IAR ARM Compiler

Make the implementation for this compiler more complete.

IAR has multiple C++ modes, historically they were reduced c++ versions
for embedded that gradually improved to the full standard (which can be
reduced again by e.g. disabling rtti and exceptions).  The new
implementation picks the best available, but the c++ mode can also be
overridden by defining `CMAKE_IAR_CXX_FLAG`.

Add C/C++ standard flags so that all modes up to and including the last
supported standard are defined.

Fixes: #16826

Help/variable/CMAKE_LANG_COMPILER_ID.rst

@@ -20,6 +20,7 @@ include:
   G95 = G95 Fortran (g95.org)
   GNU = GNU Compiler Collection (gcc.gnu.org)
   HP = Hewlett-Packard Compiler (hp.com)
+  IAR = IAR Systems (iar.com)
   Intel = Intel Compiler (intel.com)
   MIPSpro = SGI MIPSpro (sgi.com)
   MSVC = Microsoft Visual Studio (microsoft.com)