tcg-i386: Remove abort from GETPC_LDST

Indeed, remove it entirely and remove the is_tcg_gen_code check
from GETPC_EXT.

Fixes https://bugs.launchpad.net/qemu/+bug/1218098 wherein a call
to a "normal" helper function performed a sequence of tail calls
all the way into the memory helper functions, leading to a stack
frame in which the memory helper function appeared to be called
directly from tcg.

Signed-off-by: Richard Henderson <rth@twiddle.net>
