[patch] Fix makefile rule precedence so rtcl-memset.c is compiled correctly.

[Brooks Moses <bmoses at google dot com>]

So, I applied the patches that Carlos gave me for fixing the
tree-loop-distribute-patterns fun, and then ran into a rather
confusing little problem where my build got broken because
rtld-memset.os was compiled without -DNOT_IN_libc, although all the
other rtld-*.os files were compiled correctly.  (This problem actually
manifested as getting an undefined __GI_memmove symbol in
rtld-memset.os.)

It turns out that the difference is that rtld-memset.os was getting
compiled from sysdeps/powerpc/powerpc64/rtld-memset.c, whereas the
other objects were getting compiled from sources without rtld-*
prefixes.  And, in sysd-rules, we have the %.os:%.c rules coming
before the rtld-%.os:rtld-%.c rules -- which means that rtld-memset.os
first matches the %.os:%.c rule, which does not include
$(rtld-CPPFLAGS).

The attached patch is a pretty obvious fix for this, along with an
explanatory comment to prevent it from recurring.  I don't have commit
privileges, so I'd appreciate if someone could commit this for me once
it's approved.

Thanks,
- Brooks

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