toolchain/musl: simplify musl install steps

Some of these were copy/pasted from uClibc, but they are not necessary
at all for musl-libc.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@34598 3c298f89-4303-0410-b956-a3cf2f4a3e73