Turn on C99 VLAs in IAR

IAR defaults to C99 mode, but doesn't enable VLAs by default. Enable them
to make it more conformant.

We don't have much if any code using actual variable-length arrays, but
variably-modified types are occasionally used. The same switch controls
both.

(VLAs were actually already enabled in most of the project export
templates, but not the build script).

tools/export/iar_template.ewp.tmpl

@@ -402,7 +402,7 @@
         </option>
         <option>
           <name>IccAllowVLA</name>
-          <state>0</state>
+          <state>1</state>
         </option>
         <option>
           <name>IccCppDialect</name>

tools/toolchains/iar.py

@@ -40,7 +40,7 @@ class IAR(mbedToolchain):
             "-e", # Enable IAR language extension
             "--diag_suppress=Pa050,Pa084,Pa093,Pa082"],
         'asm': [],
-        'c': [],
+        'c': ["--vla"],
         'cxx': ["--guard_calls"],
         'ld': ["--skip_dynamic_initialization", "--threaded_lib"],
     }