target/arm: Move minor arithmetic helpers out of helper.c

target/arm: Move minor arithmetic helpers out of helper.c

helper.c includes some small TCG helper functions used for mostly
arithmetic instructions.  These are TCG only and there's no need for
them to be in the large and unwieldy helper.c.  Move them out to
their own source file in the tcg/ subdirectory, together with the
op_addsub.h multiply-included template header that they use.

Since we are moving op_addsub.h, we take the opportunity to
give it a name which matches our convention for files which
are not true header files but which are #included from other
C files: op_addsub.c.inc.

(Ironically, this means that helper.c no longer contains
any TCG helper function definitions at all.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20250110131211.2546314-1-peter.maydell@linaro.org
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>