ARMv8 supports tagged address, that is, the top one byte in address
is ignored.  It is always enabled on aarch64-linux.  See
https://www.kernel.org/doc/Documentation/arm64/tagged-pointers.txt

The tag in the tagged address is modeled as non-significant bits in
address, so this patch adds a new gdbarch method significant_addr_bit and
clear the non-significant bits (the top byte in ARMv8) of the virtual
address at the point before passing address to target cache layer.  IOW,
the address used in the target cache layer is already cleared.

Before this patch,
(gdb) x/x 0x0000000000411030
0x411030 <global>:	0x00000000
(gdb) x/x 0xf000000000411030
0xf000000000411030:	Cannot access memory at address 0xf000000000411030

After this patch,

(gdb) x/x 0x0000000000411030
0x411030 <global>:	0x00000000
(gdb) x/x 0xf000000000411030
0xf000000000411030:	0x00000000

Note that I used address_significant in paddress, but it causes a
regression gdb.base/long_long.exp, because gdb clears the non-significant
bits in address, but test still expects them.

p/a val.oct^M
$24 = 0x2ee53977053977^M
(gdb) FAIL: gdb.base/long_long.exp: p/a val.oct

so I defer the change there.

gdb:

2017-12-08  Yao Qi  <yao.qi@linaro.org>

	* aarch64-tdep.c (aarch64_gdbarch_init): Install gdbarch
	significant_addr_bit.
	* gdbarch.sh (significant_addr_bit): New.
	* gdbarch.c, gdbarch.h: Re-generated.
	* target.c (memory_xfer_partial): Call address_significant.
	* utils.c (address_significant): New function.
	* utils.h (address_significant): Declare.

2017-12-08  Yao Qi  <yao.qi@linaro.org>

gdb/testsuite:

	* gdb.arch/aarch64-tagged-pointer.c: New file.
	* gdb.arch/aarch64-tagged-pointer.exp: New file.