Install the btrace target ops for i386-linux-nat and amd64-linux-nat.

Install the btrace target ops for i386-linux-nat and amd64-linux-nat.

gdb/
	* amd64-linux-nat.c: Include btrace.h and linux-btrace.h.
	(amd64_linux_enable_btrace): New.
	(amd64_linux_disable_btrace): New.
	(amd64_linux_teardown_btrace): New.
	(_initialize_amd64_linux_nat): Initialize btrace ops.
	* i386-linux.nat.c: Include btrace.h and linux-btrace.h.
	(i386_linux_enable_btrace): New.
	(i386_linux_disable_btrace): New.
	(i386_linux_teardown_btrace): New.
	(_initialize_i386_linux_nat): Initialize btrace ops.
	* config/i386/linux.mh: Add linux-btrace.o.
	* config/i386/linux64.mh: Add linux-btrace.o.