Update musl to 1.1.15

[jgravelle-google]

This updates Emscripten's musl libc from 1.0.5 to 1.1.15

This was done by copying 1.0.5 into the emscripten source in order to revert all emscripten-specific changes (jgravelle-google@6993081), copying the new 1.1.15 in (jgravelle-google@3f0a2b4), and then reverting the first commit to reapply all emscripten changes at once (jgravelle-google@60dacbf). After that, all that was left to do was fix merge conflicts, and change any emscripten-specific workarounds to apply to new musl.

Of note is the assumption in newer versions of musl that __pthread_self always exists. bpowers/musl@19a1fe6 makes this explicit, and caused problems when compiling without -s USE_PTHREADS=1, because that would use library_pthread_stub.js, which used to stub __pthread_self at 0. Now we need to explicitly allocate a pthread (jgravelle-google@cc078e5) and initialize its dynamic fields (jgravelle-google@cce0512). We'd also like to make sure the whole thing is available entirely in the asm module (jgravelle-google@db9ce62) so that -s EVAL_CTORS=1 can handle calls that use it, in particular most iostream constructors rely on CURRENT_LOCALE, which is thread-specific.

This passes the entire core testsuite on my machine (for asmjs, asm2wasm, and s2wasm).