target-i386: Fix segment cache dump

When in Long Mode, cpu_x86_seg_cache() logs "DS16" because the Default
operation size bit (D/B bit) is not set for Long Mode Data Segments since
there are only Data Segments in Long Mode and no explicit 16/32/64-bit
Descriptors.
This patch fixes this by checking the Long Mode Active bit of the hidden
flags variable and logging "DS" if it is set. (I.e. in Long Mode all Data
Segments are logged as "DS")

Signed-off-by: Tobias Markus <tobias@markus-regensburg.de>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>