configure: Add -mstackrealign flags to CFLAGS on OS/2

configure: Add -mstackrealign flags to CFLAGS on OS/2

Many codes require -mstackrealign flags. Although -mstackrealign has
been already added to CFLAGS of some modules, SIGSEGV occurs in other
modules than those modules.

The best way may be to find causes and to fix them. However, we
cannot know those causes until SIGSEGV occur really. In addition, if
SIGSEGV occurs in other programs, it will be fatal.

So adding -mstackrealign flags to CFLAGS unconditionally is
reasonable.

Change-Id: I999ef597a6afe97f5e7cc7bffaa866537c3eedd2