Fix musl SONAME (#4762)

Without this, our default `musl` in the BB rootfs has the SONAME `libc.musl-.so.1`:

```
# readelf -d /lib/libc.musl-x86_64.so.1 

Dynamic section at offset 0x94928 contains 18 entries:
  Tag        Type                         Name/Value
 0x000000000000000e (SONAME)             Library soname: [libc.musl-.so.1]
```

M/Musl/build_tarballs.jl

@@ -29,7 +29,7 @@ musl_arch()
     esac
 }
 
-export LDFLAGS="${LDFLAGS} -Wl,-soname,libc.musl-$(musl_target).so.1"
+export LDFLAGS="${LDFLAGS} -Wl,-soname,libc.musl-$(musl_arch).so.1"
 ${WORKSPACE}/srcdir/musl-*/configure --prefix=/usr \
     --build=${MACHTYPE} \
     --host=${target} \