tcg/ppc: Use TCG_REG_TMP2 for scratch tcg_out_qemu_st

tcg/ppc: Use TCG_REG_TMP2 for scratch tcg_out_qemu_st

In the fallback when STDBRX is not available, avoid clobbering
TCG_REG_TMP1, which might be h.base, which is still in use.
Use TCG_REG_TMP2 instead.

Cc: qemu-stable@nongnu.org
Fixes: 01a112e2e9 ("tcg/ppc: Reorg tcg_out_tlb_read")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-By: Michael Tokarev <mjt@tls.msk.ru>
(cherry picked from commit 4cabcb89b101942346aebff081aa1453e958fe7f)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>