Add an xfail manifest for x86_64-unknown-linux-gnu to trunk.

Add an xfail manifest for x86_64-unknown-linux-gnu to trunk.

This patch adds an xfail manifest for trunk for x86_64 builds. I find
this useful to determine whether my patch has introduced new failures.
The failures in these manifest are always present in trunk and
deciding what to ignore is not very straightforward.

I will keep maintaining this manifest out of clean builds. They are
not hard to maintain. Manifest files can be generated by going to the
top of the build directory and typing:

$ cd <top-of-bld-dir>
$ <path-to-src>/contrib/testsuite-management --produce_manifest

This will generate a .xfail file with the triple name of the target
you just built.  Once this file exist you can run the validator again
on the build directory with no arguments.  It should produce the
output:

$ cd <top-of-bld-dir>
$ <path-to-src>/contrib/testsuite-management/validate_failures.py
Source directory: <path-to-src>
Build target:     x86_64-unknown-linux-gnu
Manifest:         <path-to-src>/contrib/testsuite-management/x86_64-unknown-linux-gnu.xfail
Getting actual results from build
        ./x86_64-unknown-linux-gnu/libstdc++-v3/testsuite/libstdc++.sum
        ./x86_64-unknown-linux-gnu/libffi/testsuite/libffi.sum
        ./x86_64-unknown-linux-gnu/libgomp/testsuite/libgomp.sum
        ./x86_64-unknown-linux-gnu/libgo/libgo.sum
        ./x86_64-unknown-linux-gnu/boehm-gc/testsuite/boehm-gc.sum
        ./x86_64-unknown-linux-gnu/libatomic/testsuite/libatomic.sum
        ./x86_64-unknown-linux-gnu/libmudflap/testsuite/libmudflap.sum
        ./x86_64-unknown-linux-gnu/libitm/testsuite/libitm.sum
        ./x86_64-unknown-linux-gnu/libjava/testsuite/libjava.sum
        ./gcc/testsuite/g++/g++.sum
        ./gcc/testsuite/gnat/gnat.sum
        ./gcc/testsuite/ada/acats/acats.sum
        ./gcc/testsuite/gcc/gcc.sum
        ./gcc/testsuite/gfortran/gfortran.sum
        ./gcc/testsuite/obj-c++/obj-c++.sum
        ./gcc/testsuite/go/go.sum
        ./gcc/testsuite/objc/objc.sum

SUCCESS: No unexpected failures.

If the output shows new failures, you investigate them. If they are
not yours, you can add them to the xfail manifest (after reporting
them) and then commit the modified .xfail file.

Long term, I would like to have this script pull manifest files from
postings made to gcc-testresults. This way, we won't have to maintain
these .xfail files manually. In branches this is not a big problem,
but in trunk it may be a tad annoying.

From-SVN: r190404