Remove various rarely used test targets from the generated Makefile.

Remove these targets and their documentation:

check.third check.utf16 check.utf8 coretest minitest.utf16 test.deparse
test.taintwarn test.third test.torture test.utf16 test.utf8
test_notty.deparse test_notty.third test_prep.third torturetest ucheck
ucheck.third ucheck.utf16 ucheck.valgrind utest utest.third utest.utf16
utest.valgrind

It's still possible to run the actions these targets "by hand", if desired.
This commit simply removes the convenience targets from the Makefile,
reducing its complexity.
Remove Makefile targets and tools related to Irix and Tru64 debugging tools.

Remove the targets:

perl.pixie perl.pixie.atom perl.pixie.config perl.pixie.irix perl.third
perl.third.config

It's still possible to run the actions these targets "by hand", if desired.
This commit removes the convenience targets from the Makefile, reducing its
complexity. It also removes the related support scripts testall.atom and
thirdclean from Porting/

pixie is a performance analysis tool for Irix and Tru64
Third Degree is a memory checker tool for Tru64

Given that Tru64 went out of support at the end of 2012, and Irix goes out
of support at the end of 2013, it's very unlikely that anyone is still
actively profiling or debugging perl on either platform, and hence using
these targets. It's been several years since we've even had a regular bug
report from either platform.
Inline the Makefile target minitest.prep into its only user.

This is strictly a refactoring, so do not change the rules themselves, despite
them being ugly and fragile.
valgrind doesn't require that perl was built with -g, so remove the check.

C<make test.valgrind> will run quite happily on a perl built with
optimisation and without debugging symbols. So don't enforce -g.
We don't actually need to set $ENV{PERL} for the tests to work.

Whatever the executable is named at the top level, it's always symlinked
as ./perl in t, so there's no need to set an environment variable to
override the expected name.
Remove the explicit purify/quantify/purecov targets and documentation.

It's not clear whether IBM still sell quantify or purecov. They still seem
to sell purify, but I'm not sure if anyone is using it these days to detect
bugs in perl.

This doesn't prevent anyone from using these tools if they have them, as
it's still possible to run the commands by "hand". But by removing probably
unused code and documentation, the signal to noise ratio improves.
In perlhacktips, no need to give 3 ways to set an environment variable.
Update perlhacktips to note that Address Sanitizer is now also in gcc 4.8

Also s/linux/Linux/ in two places.
Run Porting/podtidy on pod/perlhacktips.pod
Eliminate the perl.gprof and perl.gcov Makefile targets.

I don't feel that it's worthwhile having specific named targets for
building named binaries for use with gprof and gcov given that one has to
(re)Configure with the appropriate C compiler flags, hence all the object
files and the F<perl> that the build tree would build are just as enabled
(or contaminated) with profiling code as the specially named binary.

Update the documentation on using gprof and gcov to reflect that the binary
named F<perl> is now the binary that is profiled.