tcg: Fix occasional TCG broken problem when ldst optimization enabled

tcg: Fix occasional TCG broken problem when ldst optimization enabled

is_tcg_gen_code() checks the upper limit of TCG generated code range wrong, so
that TCG could get broken occasionally only when CONFIG_QEMU_LDST_OPTIMIZATION
enabled. The reason is code_gen_buffer_max_size does not cover the upper range
up to (TCG_MAX_OP_SIZE * OPC_BUF_SIZE), thus code_gen_buffer_max_size should be
modified to code_gen_buffer_size.

This patch has been already applied to mainstream QEMU.
So it should be overwritten when applying QEMU version more than 1.3 in future.

Signed-off-by: Yeongkyoon Lee <yeongkyoon.lee@samsung.com>