musl: clean up more libc/math/ files

Clean up all remaining libc/math/ files which we can take from musl.
Some of these files we didn't modify at all (just took a slightly
different version), and some we modified to avoid unjustified warnings.

Now, instead of modifying the musl files to avoid errors (unused variable,
and false "maybe uninitialized" errors), I added in libc/build.mk explicit
"-Wno-error" directives to warn, not stop compilation, when the known
issues happen on the known files (e.g., maybe-uninitialized on lgammar_r.c).
Hopefully this won't be necessary in newer version of musl.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>