Add a disassembly flag to omit the pc prefix and use it in the "record

Add a disassembly flag to omit the pc prefix and use it in the "record
instruction-history" command of record-btrace.

The pc prefix would appear multiple times in the branch trace disassembly,
which is more confusing than helpful.

gdb/
	* record-btrace.c (btrace_insn_history): Omit the pc prefix in
	the instruction history disassembly.
	* disasm.c (dump_insns): Omit the pc prefix, if requested.
	* disasm.h (DISASSEMBLY_OMIT_PC): New.