Fix build with GNU Make 3.81

Fix build with GNU Make 3.81

GNU Make 3.81 is apparently confused when the same
source file is processed by a pattern rule and an
explicit rule at the same time with different output file.
The pattern %.o: ../%.c and alloc-ipa.o: ../alloc.c
both have the source ../alloc.c but two independent
object files alloc.o and alloc-ipa.o, so
while building gdbserver I see the following message:

make[4]: Circular alloc-ipa.o <- ../alloc.c dependency dropped.
  CXX    alloc-ipa.o
g++: warning: '-x c++' after last input file has no effect
g++: fatal error: no input files
compilation terminated.

In the make debug output I see the pattern is first correct:

alloc-ipa.o: ../alloc.c | config.h build-gnulib-gdbserver/import/string.h
        $(IPAGENT_COMPILE) $(WARN_CFLAGS_NO_FORMAT) $<
        $(POSTCOMPILE)

But after the "Circular" dependency is dropped, the pattern
is changed to:

alloc-ipa.o: | config.h build-gnulib-gdbserver/import/string.h
        $(IPAGENT_COMPILE) $(WARN_CFLAGS_NO_FORMAT) $<
        $(POSTCOMPILE)

So indeed now $< is empty, and the build step fails.

This happens only when alloc.o needs to be built, when alloc.o
was already built, the build succeeds, but it takes often
several attempts until the build succeeds.
By rewriting the alloc-ipa.c: ../alloc.c rule into a pattern
rule, the problem goes away.

While already at it, this patch removes also the
$(WARN_CFLAGS_NO_FORMAT) from the build rule, which is just a
copy/paste thing that is not necessary for alloc.c at all.