Don't call clock_getres in sysconf.

There are a hundred other reasons why we can't run on kernels old enough
to not have all the clocks covered by sysconf.

This was causing trouble for jemalloc 4.1.0 in a seccomp-constrained process
because jemalloc 4.1.0 introduced a call to sysconf that caused us to make
clock_getres syscalls for the first time, leading to SIGSYS.

Bug: http://b/27408522
Change-Id: I2eb6986d871bc03cbef278e5617734409c39e057