Thanks, this indeed looks like a bug in the logic that prevents the incompatible armclang options from being passed to armasm. We have fixed it in the following build: VisualGDB-5.6.109.4816.msi.
You can also apply the patch on your side without reinstalling VisualGDB by locating the following statement in the %VISUALGDB_DIR%\MSBuild\Targets\gcc.targets file:
<ItemGroup Condition="'$(GNUToolchainSubtype)' == 'armclang'">
<ClCompile Condition="'%(ClCompile.IsAssemblyFile)' == 'true'">
<AdditionalOptions/>
</ClCompile>
</ItemGroup>
Then, you would need to change the <AdditionalOptions/> line to this:
<AdditionalOptions>$(ARMASMArguments)</AdditionalOptions>