Simplify tic6x and s390x expedite registers

Nowadays, we have six tic6x expedite registers, which are duplicated.

tic6x-c64xp-expedite = A15,PC
tic6x-c64x-expedite = A15,PC
tic6x-c62x-expedite = A15,PC
tic6x-c64xp-linux-expedite = A15,PC
tic6x-c64x-linux-expedite = A15,PC
tic6x-c62x-linux-expedite = A15,PC

in features/Makefile, we have

   echo "expedite:$(if $($*-expedite),$($*-expedite),$($(firstword $(subst -, ,$(notdir $*)))-expedite))" \
        >> $(outdir)/$*.tmp

which means for a given bar/foo-baz.xml, we'll look for either
bar/foo-baz-expedite or foo-expedite.  We can define only one generic
expedite register for all different ti6cx and s390x target descriptions.
Actually, we've done that for x86 target descriptions.

Re-run 'make GDB=/path/build/gdb all' to regenerate regformats/*.dat files,
and they are not changed.

gdb:

2017-10-13  Yao Qi  <yao.qi@linaro.org>

	* features/Makefile: Remove tic6x-*-expedite, add tic6x-expedite.
	Remove s390x-*-expedite, add s390x-expedite.