Put crate metadata first in rlibs whenever possible

[bjorn3]

In the past rlib metadata had to be the last archive member as the linker stops at the first non-object member. The rlib metadata has somewhat recently been changed to be wrapped in an object file on most platforms, which allows it to be anywhere in the rlib. Putting it first would allow for quicker metadata reading as it would no longer be necessary to read the header of every archive member to find the crate metadata, only the first member header.