FreeBSD 9.0/10.0 amd64 (CLANG): mysterious client crahses when typing left/right arrow key

Since yesterday after a make world on both FreeBSD versions I realize a 
strange behaviour in several GTK clients (gq for instance, freshly 
installed yesterday) and Firefox and Thunderbird (they have not been 
recompiled now for days).

This behaviour occured earlier this year on FreeBSD 9.0-CURRENT, around 
July. I tried to install FreeBSD Current compiled with CLANG on a 
Core-i5 driven notebook with option --mtune=native, --mtune=i7 or even 
avoid using --mtune= and then it occured that the whole system was crap, 
since typing of TAB key immediately exited the shell or typing the arrow 
keys resulted in the same.

This happened NOT when system got compiled with --mtune=core2 on that 
Core-i5 driven Notebook (Dell Latitude E6510)!

I didn't test this behaviour on that notebook yet, but whill this night.

The notebook have had a GENERIC kernel that time when the problem 
occured, now I use on all boxes custom kernel and these lines seem to be 
specificially for the terminal emulation:

# Console options
options         MAXCONS=8               # maximum No. of consoles
options         SC_ALT_MOUSE_IMAGE      # simplified mouse cursor in 
text mode
options         SC_DFLT_FONT            # compile font in
makeoptions     SC_DFLT_FONT=cp850
options         SC_DISABLE_KDBKEY       # disable `debug' key
options         SC_DISABLE_REBOOT       # disable reboot key sequence
options         SC_HISTORY_SIZE=512     # number of history buffer lines
#options        SC_MOUSE_CHAR=0x3       # char code for text mode mouse 
cursor
options         SC_PIXEL_MODE           # add support for the raster 
text mode
options         SC_NORM_ATTR=(FG_GREEN|BG_BLACK)
options         SC_NORM_REV_ATTR=(FG_YELLOW|BG_GREEN)
options         SC_KERNEL_CONS_ATTR=(FG_RED|BG_BLACK)
options         SC_KERNEL_CONS_REV_ATTR=(FG_BLACK|BG_RED)
#options        SC_CUT_SPACES2TABS      # convert leading spaces into tabs
#options        SC_CUT_SEPCHARS=\"x09\" # set of characters that delimit 
words
                                         # (default is single space - 
\"x20\")
                                         #options        SC_TWOBUTTON_MOUSE

# Enable experimental features of the syscons terminal emulator (teken).
options         TEKEN_UTF8              # UTF-8 output handling
#options                TEKEN_CONS25            # cons25-style terminal 
emulation

# Enable VESA
#options                X86BIOS
#options                VESA


Any idea?

Regards,
Oliver