Support for DTrace USDT probes in x86_64 targets.

Support for DTrace USDT probes in x86_64 targets.

This patch adds the target-specific code in order to support the
calculation of DTrace probes arguments in x86_64 targets, and also the
enabling and disabling of probes.  This is done by implementing the
`dtrace_*' gdbarch handlers.

gdb/ChangeLog:

2015-02-17  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* amd64-linux-tdep.c: Include "parser-defs.h" and "user-regs.h".
	(amd64_dtrace_parse_probe_argument): New function.
	(amd64_dtrace_probe_is_enabled): Likewise.
	(amd64_dtrace_enable_probe): Likewise.
	(amd64_dtrace_disable_probe): Likewise.
	(amd64_linux_init_abi): Register the
	`gdbarch_dtrace_probe_argument', `gdbarch_dtrace_enable_probe',
	`gdbarch_dtrace_disable_probe' and
	`gdbarch_dtrace_probe_is_enabled' hooks.
	(amd64_dtrace_disabled_probe_sequence_1): New constant.
	(amd64_dtrace_disabled_probe_sequence_2): Likewise.
	(amd64_dtrace_enable_probe_sequence): Likewise.
	(amd64_dtrace_disable_probe_sequence): Likewise.