This drops the `.cpu` directive from the ARM gchelper_*.s files. Having
this directive breaks the linker when targeting older CPUs (e.g. `-mthumb
-mthumb-interwork` for `-mcpu=arm7tdmi`). The actual target CPU should be
determined by the compiler options.
The exact CPU doesn't actually matter, but rather the supported assembly
instruction set. So the files are renamed to *_thumb1.s and *thumb2.s to
indicate the instruction set support instead of the CPU support.
Signed-off-by: David Lechner <david@pybricks.com>
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