Add #defines for some key enums.

Building ruby actually trips over both of these:

* if the RTLD_ constants aren't #defined, it uses its own incorrect values.

* if the REG_ constants aren't #defined, it confuses x86 with x86-64.

In all other places where we have enums in our headers, we already match
existing glibc practice.

Bug: http://b/62531921
Test: builds
Change-Id: I5b3aab25a1a24611bdc58f2eda4104a78e9f841c