Remove the dependency on the non-uapi __kernel_nlink_t.

The kernel doesn't have an nlink_t; it just uses the equivalent of
uint32_t. We already had a usable __nlink_t in the C library, so
let's just define our nlink_t in terms of __nlink_t, which is what
__nlink_t was meant for anyway.

Note that our struct stat just follows the kernel, and doesn't refer
to nlink_t anyway.

Change-Id: I2a56e418e42404b1741b08c50554b03c11caebae
Add a temporary uapi/linux/compiler.h until upstream has one.

Bug: 11559337
Change-Id: I205974b1a26e4f85d0e11503afa80669195d211c