[AArch64] Rename boolean arguments in decoding functions

[AArch64] Rename boolean arguments in decoding functions

This patch cleans up the decoding functions using booleans when they can
decode two instructions.  The boolean argument is used to know which of
the two instructions was decoded.

The instructions affected are BR/BLR, B/BL, CBZ/CBNZ and TBZ/TBNZ.

These arguments would be named after a named bit in the instruction
encoding, this patch renames them to 'is_XXX'.  Furthermore, the
'unsigned' type would be used to describe a boolean while
aarch64_decode_cb would use 'int' (see the 'is64' argument).  This patch
makes all booleans be 'int' and decoded bitfields be 'unsigned'.

gdb/ChangeLog:

	* aarch64-tdep.c (decode_b): Rename link argument to is_bl.
	Change its type to int *.
	(decode_br): Rename link argument to is_blr.  Change its type to
	int *.
	(decode_cb): Rename op argument to is_cbnz.  Change its type to
	int *.
	(decode_tb): Rename op argument to is_tbnz.  Change its type to
	int *.  Set is_tbnz to either 1 or 0.
	(aarch64_analyze_prologue): Change type of is_link to int.  Add
	new variables is_cbnz and is_tbnz.  Adjust call to
	aarch64_decode_cb and aarch64_decode_tb.