Reland "[lldb][test] Only add -m(64|32) for GCC on non Arm/AArch64 platforms"

This reverts commit fd5206cc.

Fixing the test case would require some awkard %if use that I'm not
sure would even work, or splitting it into 2 copies that are almost
identical.

Instead, always add -m for clang, which allows it for all targets,
but not for gcc which does not.