target/arm: Use tcg_constant for do_brk{2,3}

In these cases, 't' did double-duty as zero source and
temporary destination.  Split the two uses and narrow
the scope of the temp.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20220426163043.100432-47-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>