i#7842: ISA feature AARCH64 bug fix

[edeiana]

Fixes instr_get_isa_feature() in AARCH64 where instructions like bcond
were incorrectly reported with an <invalid> ISA feature in the opcode_mix tool.
The root cause was that instr_get_isa_feature() instantiated a decode_info_t
struct without initialization, leaving the check_reachable flag containing
indeterminate stack data.
When this flag happened to be true, the encoding walk of instr_get_isa_feature()
enforced reachability checks on PC-relative operands.
Depending on how instr_get_isa_feature() is used, pc is not necessarily the
instruction's actual pc, it can be an unrelated address like a buffer, so disabling
the reachability check is necessary.

Adds a unit test that would trigger returning <invalid> ISA feature if the
reachability check were to be enabled.

Issue #7842