Fix race when building ada-lex.c

Fix race when building ada-lex.c

Prevent a race when building ada-lex.c, and any target of rules .c:.l or
.c:.y.  The target should be written only at the last step, else SIGINT
(^C) can leave an inconsistent state.  Being .PRECIOUS makes it even
worse.

gdb/ChangeLog:

	PR build/22873
	* gdb/Makefile.in: (.c:.l, .c:.y): Write the target only in the
	last step, and do it atomically.