Implement branch tracing on Linux based on perf_event such that it can be shared

Implement branch tracing on Linux based on perf_event such that it can be shared
between gdb and gdbserver.

gdb/
	* common/linux_btrace.h: New file.
	* common/linux_btrace.c: New file.
	* Makefile.in (SFILES): Add btrace.c.
	(HFILES_NO_SRCDIR): Add common/linux-btrace.h.
	(COMMON_OBS): Add btrace.o.
	(linux-btrace.o): New rule.

gdbserver/
	* Makefile.in (SFILES): Add $(srcdir)/common/linux-btrace.c.
	(linux_btrace_h): New variable.
	(linux-btrace.o): New rule.