class readable_regcache and pass readable_regcache to gdbarch pseudo_register_read...

class readable_regcache and pass readable_regcache to gdbarch pseudo_register_read and pseudo_register_read_value

pseudo registers are either from raw registers or memory, so
gdbarch methods pseudo_register_read and pseudo_register_read_value
should have regcache object which only have read methods.  In other
words, we should disallow writing to regcache in these two gdbarch
methods.  In order to apply this restriction, this patch adds a new
class readable_regcache, derived from reg_buffer, and it only has
raw_read and cooked_read methods.  regcache is derived from
readable_regcache.  This patch also passes readable_regcache instead of
regcache to gdbarch methods pseudo_register_read and
pseudo_register_read_value.

This patch moves raw_read* and cooked_read* methods to readable_regcache,
which is straightforward.  One thing not straightforward is that I split
regcache::xfer_part to readable_regcache::read_part and regcache::write_part,
because readable_regcache can only have methods to read.

readable_regcache is an abstract base class, and it has a pure virtual
function raw_update, because I don't want readable_regcache know where
these raw registers are from.  They can be from either the target
(readwrite regcache) or the regcache itself (readonly regcache).

gdb:

2018-02-21  Yao Qi  <yao.qi@linaro.org>

	* aarch64-tdep.c (aarch64_pseudo_register_read_value): Change
	parameter type to 'readable_regcache *'.
	* amd64-tdep.c (amd64_pseudo_register_read_value): Likewise.
	* arm-tdep.c (arm_neon_quad_read): Likewise.
	(arm_pseudo_read): Likewise.
	* avr-tdep.c (avr_pseudo_register_read): Likewise.
	* bfin-tdep.c (bfin_pseudo_register_read): Likewise.
	* frv-tdep.c (frv_pseudo_register_read): Likewise.
	* gdbarch.c: Re-generated.
	* gdbarch.h: Re-generated.
	* gdbarch.sh (pseudo_register_read): Change parameter type to
	'readable_regcache *'.
	(pseudo_register_read_value): Likewise.
	* h8300-tdep.c (pseudo_from_raw_register): Likewise.
	(h8300_pseudo_register_read): Likewise.
	* hppa-tdep.c (hppa_pseudo_register_read): Likewise.
	* i386-tdep.c (i386_mmx_regnum_to_fp_regnum): Likewise.
	(i386_pseudo_register_read_into_value): Likewise.
	(i386_pseudo_register_read_value): Likewise.
	* i386-tdep.h (i386_pseudo_register_read_into_value): Update
	declaration.
	* ia64-tdep.c (ia64_pseudo_register_read): Likewise.
	* m32c-tdep.c (m32c_raw_read): Likewise.
	(m32c_read_flg): Likewise.
	(m32c_banked_register): Likewise.
	(m32c_banked_read): Likewise.
	(m32c_sb_read): Likewise.
	(m32c_part_read): Likewise.
	(m32c_cat_read): Likewise.
	(m32c_r3r2r1r0_read): Likewise.
	(m32c_pseudo_register_read): Likewise.
	* m68hc11-tdep.c (m68hc11_pseudo_register_read): Likewise.
	* mep-tdep.c (mep_pseudo_cr32_read): Likewise.
	(mep_pseudo_cr64_read): Likewise.
	(mep_pseudo_register_read): Likewise.
	* mips-tdep.c (mips_pseudo_register_read): Likewise.
	* msp430-tdep.c (msp430_pseudo_register_read): Likewise.
	* nds32-tdep.c (nds32_pseudo_register_read): Likewise.
	* regcache.c (regcache::raw_read): Move it to readable_regcache.
	(regcache::cooked_read): Likewise.
	(regcache::cooked_read_value): Likewise.
	(regcache_cooked_read_signed):
	(regcache::cooked_read): Likewise.
	* regcache.h (readable_regcache): New class.
	(regcache): Inherit readable_regcache.  Move some methods to
	readable_regcache.
	* rl78-tdep.c (rl78_pseudo_register_read): Change
	parameter type to 'readable_regcache *'.
	* rs6000-tdep.c (do_regcache_raw_read): Remove.
	(e500_pseudo_register_read): Change parameter type to
	'readable_regcache *'.
	(dfp_pseudo_register_read): Likewise.
	(vsx_pseudo_register_read): Likewise.
	(efpr_pseudo_register_read): Likewise.
	* s390-tdep.c (s390_pseudo_register_read): Likewise.
	* sh-tdep.c (sh_pseudo_register_read): Likewise.
	* sh64-tdep.c (pseudo_register_read_portions): Likewise.
	(sh64_pseudo_register_read): Likewise.
	* sparc-tdep.c (sparc32_pseudo_register_read): Likewise.
	* sparc64-tdep.c (sparc64_pseudo_register_read): Likewise.
	* spu-tdep.c (spu_pseudo_register_read_spu): Likewise.
	(spu_pseudo_register_read): Likewise.
	* xtensa-tdep.c	(xtensa_register_read_masked): Likewise.
	(xtensa_pseudo_register_read): Likewise.