[musl] Use 'j' suffix rather than 'i' suffix in _Complex_I definition

Both the 'i' and 'j' suffixes on a floating-point literal to mean
"imaginary" are GNU extensions. But ToT Clang (r310423) has stopped
accepting 'i' in non-GNU -std= modes, while it still accepts 'j'.

Change-Id: If76ce3176f0bd2e38f625657c68417bd9e4708a9

[musl] Use #pragma once in musl's public headers

Like the prior commit, this only replaces existing ifdef guards.

Change-Id: I39be129111db9f129aef72a53f986ce6a49376ac

[musl] clang format musl

Change-Id: I3bc6ab7655410514691cd07f08454f3afd41d9b3

[magenta] Initial commit