compile part of musl to compensate for bionic lacking symbols

fix #14 (closed)

I tested on an API 22 emulator to make sure the changes indeed work as intended.

How to review this:
I copied most of the code verbatim from musl v1.2.3 (7a43f6fea9081bdd53d8a11cef9e9fab0348c53d).
The exception being the central file importing the rest: crates/onionmasq-mobile/bionic-missing/all-missing.c.
Most files from musl are unchanged. For some, I commented imports which we did not actually need, and were bringing even more of musl.
It follows that most .h and .c files shouldn't really be reviewed (except for all-missing.c), and can be ignored or diffed with their musl counterpart.