x86: turn on -Wl,--fatal-warnings

In 4f2f16da, we added
-Wl,--warn-shared-textrel to TARGET_GLOBAL_LDFLAGS. This generated
a warning whenever we created a shared library with a text
relocation.  At the time, we wrote:

=====
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)
=====

All code which uses text relocations has either been fixed, or
we've temporarily worked around the issue so the code can still
compile.

Enable --fatal-warnings.

Bug: 11353056
Change-Id: I2f2c65352166ef0a96f54858de1556f0290fe306