Go packages use build tags (see the section on Build Constraints at
https://golang.org/pkg/go/build/) to select which files to build on
specific systems.

Previously the libgo Makefile explicitly listed the set of files to
compile for each package.  For packages that use build tags, this
required a lot of awkward automake conditionals in the Makefile.

This patch changes the build to look at the build tags in the files.
The new shell script libgo/match.sh does the matching.  This required
adjusting a lot of build tags, and removing some files that are never
used.  I verified that the exact same sets of files are compiled on
x86_64-pc-linux-gnu.  I also tested the build on i386-sun-solaris
(building for both 32-bit and 64-bit).

Writing match.sh revealed some bugs in the build tag handling that
already exists, in a slightly different form, in the gotest shell
script.  This patch fixes those problems as well.

The old code used automake conditionals to handle systems that were
missing strerror_r and wait4.  Rather than deal with those in Go,
those functions are now implemented in runtime/go-nosys.c when
necessary, so the Go code can simply assume that they exist.

The os testsuite looked for dir_unix.go, which was never built for
gccgo and has now been removed.  I changed the testsuite to look for
dir.go instead.

Note that if you have an existing build directory, you will have to
remove all the .dep files in TARGET/libgo after updating to this
patch.  There isn't anything that will force them to update
automatically.

Bootstrapped on x86_64-pc-linux-gnu and i386-sun-solaris.  Ran Go
testsuite on x86_64-pc-linux-gnu.  Committed to mainline.

Ian