Consolidate API of target_supports_multi_process

Consolidate API of target_supports_multi_process

This simple commit consolidates the API of
target_supports_multi_process.  Since both GDB and gdbserver use the
same function prototype, all that was needed was to move create this
prototype on gdb/target/target.h and turn the macros declared on
gdb/{,gdbserver/}target.h into actual functions.

Regtested (clean pass) on the BuildBot.

gdb/ChangeLog:
2016-10-06  Sergio Durigan Junior  <sergiodj@redhat.com>

	* target.c (target_supports_multi_process): New function, moved
	from...
	* target.h (target_supports_multi_process): ... here.  Remove
	macro.
	* target/target.h (target_supports_multi_process): New prototype.

gdb/gdbserver/ChangeLog:
2016-10-06  Sergio Durigan Junior  <sergiodj@redhat.com>

	* target.c (target_supports_multi_process): New function, moved
	from...
	* target.h (target_supports_multi_process): ... here.  Remove
	macro.