Musl stdio

Migrates several libc stdio functions to use musl:

sscanf, sprintf, asprintf, vsnprintf, vsprintf, vasprintf, and the unctions frexp, rexpf, frexpl which the printf/scanf ones call to.

The functions scanf, fscanf, printf and fprintf which are directly tied to the file system have not been migrated, since we are not using the musl filesystem.

Other tests pass, but the test `test_cases` starts to fail after this, because the .ll files in those tests depend on the JavaScript-implemented functions and they don't link to the musl libc. What do you think would be a good way to handle those @kripken ?