libc: Move <sgidefs.h> to libc/arch-mips/

This is a MIPS-only header, and should not be part of the common
headers. See http://b.android.com/79841 for context.

Change-Id: I610bc3ff626b57e7854dad15a4a2f67e1e5ded75
Merge "libc: Move <sgidefs.h> to libc/arch-mips/"