Allow the target to set MAX_BITSIZE_MODE_ANY_MODE

Allow the target to set MAX_BITSIZE_MODE_ANY_MODE

The default value of MAX_BITSIZE_MODE_ANY_MODE is calculated
from the initial mode sizes specified in the modes.def file.
The target needs to be able to override it if ADJUST_BYTESIZE
& co. can choose a bigger size.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* doc/rtl.texi (MAX_BITSIZE_MODE_ANY_MODE): Describe how the default
	is calculated and how it can be overridden.
	* genmodes.c (max_bitsize_mode_any_mode): New variable.
	(create_modes): Initialize it from MAX_BITSIZE_MODE_ANY_MODE,
	if defined.
	(emit_max_int): Use it to set the output MAX_BITSIZE_MODE_ANY_MODE,
	if nonzero.

From-SVN: r256206