Currently, some fixed point operations (notably multiplication) fail to
compile when used in Thumb-mode routines. This occurs because GCC
attempts to inline the operation into the Thumb-mode routine, but the
operation uses ARM-mode only instructions. This commit adds the ".arm"
directive into the inline assembly of the implementation, which informs
GCC that the assembly uses ARM-mode instructions and prevents inlining.
As a result, fixed point numbers can be used from both ARM-mode and
Thumb-mode code without issues! Usage in ARM-mode should still be
preferred for optimal performance though.
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