Pass readable_regcache to gdbarch method read_pc

We can pass readable_regcache to gdbarch method read_pc where it is
allowed to do read from regcache.

gdb:

2018-01-30  Yao Qi  <yao.qi@linaro.org>

	* avr-tdep.c (avr_read_pc): Change parameter type to
	readable_regcache.
	* gdbarch.sh (read_pc): Likewise.
	* gdbarch.c: Re-generated.
	* gdbarch.h: Re-generated.
	* hppa-tdep.c (hppa_read_pc): Change parameter type to
	readable_regcache.
	* ia64-tdep.c (ia64_read_pc): Likewise.
	* mips-tdep.c (mips_read_pc): Likewise.
	* spu-tdep.c (spu_read_pc): Likewise.