Fix our <complex.h> support.

We build libm with -fvisibility=hidden, so we weren't exporting any
of the <complex.h> functions.

We also weren't building many of the functions anyway.

We were also missing the complex inverse trigonometric functions.

And because we didn't even have perfunctory "call each function once"
tests, we didn't notice that we weren't exporting any symbols, so this
patch adds at least that level of testing.

Change-Id: Ibcf2843f507126c51d134cc5fc8d67747e033a0d