x86: add --warn-shared-textrel

Emit a warning when the static linker detects a shared library
has text relocations. Text relocations make it harder to
share pages across processes, and make it harder to use
certain memory protection features in, for example, SELinux.

This warning will turn into an error in a future change
(via --fatal-warnings)

Change-Id: I2d169c7ce3600d02e25591f3ec15aba363730298
References: http://www.akkadia.org/drepper/textrelocs.html