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

[patch attached]