Makefile: Replace old suffix rules with pattern rules

Makefile: Replace old suffix rules with pattern rules

As mentioned here [1], suffix rules are obsolete and have been
superseeded with pattern rules.  People (myself included, before writing
this patch) are more likely to know what pattern rules are than suffix
rules.

AFAIK, .SUFFIXES targets are only used for those rules, and can be
removed as well.

New in v2:

  - Replace rule in gdbserver/Makefile.in as well.

[1] https://www.gnu.org/software/make/manual/html_node/Suffix-Rules.html

gdb/ChangeLog:

	* Makefile.in (.c.o): Replace rule with ...
	(%.o: %.c): ... this one.
	(.po.gmo): Replace rule with ...
	(%.gmo: %.po): ... this one.
	(.po.pox): Replace rule with ...
	(%.pox: %.po): ... this one.
	(.y.c): Replace rule with ...
	(%.c: %.y): ... this one.
	(.l.c): Replace rule with ...
	(%.c: %.l): ... this one.
	(.SUFFIXES): Remove all instances.

gdb/gdbserver/ChangeLog:

	* Makefile.in (.c.o): Replace rule with ...
	(%.o: %.c): ... this one.