base: make boot_clock work on host linux, hide it on non-linux.

boot_clock was previously returning zero on any platform that doesn't
define __ANDROID__, including host bionic. Instead of returning a bogus
value, just hide it on non-Linux platforms.

Bug: http://b/37758947
Test: libbase_test32/64 on linux
Change-Id: I96e1d8b92dc44c6308408900cf0d27e1e7db5569