auto merge of #8602 : sanxiyn/rust/sysconf, r=graydon

Linux and Android share the kernel, but not the C library, so sysconf constants are different. For example, _SC_PAGESIZE is 30 on Linux, but 39 on Android.

This patch
* splits sysconf constants to sysconf module
* merges non-MIPS and MIPS sysconf constants (they are same)
* adds Android sysconf constants

This patch also lets mmap tests to pass on Android.

Author: bors