tcg: Document tcg_qemu_tb_exec() and provide constants for low bit uses

tcg: Document tcg_qemu_tb_exec() and provide constants for low bit uses

Document tcg_qemu_tb_exec(). In particular, its return value is a
combination of a pointer to the next translation block and some
extra information in the low two bits. Provide some #defines for
the values passed in these bits to improve code clarity.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
(cherry picked from commit 0980011b4f66482d2733ab2dd0f2f61747772c6b)

Conflicts:
	tcg/tcg.h

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>