Refactor linker script generation

The generation of linker scripts was badly balanced, as all sorts of
platform dependent stuff went into the top build.info, when that part
should really be made as simply and generic as possible.

Therefore, we move a lot of the "magic" to the build files templates,
since they are the place for platform dependent things.  What remains
is to parametrize just enough in the build.info file to generate the
linker scripts correctly for each associated library.

"linker script" is a term usually reserved for certain Unix linkers.
However, we only use them to say what symbols should be exported, so
we use the term loosely for all platforms.  The internal extension is
'.ld', and is changed by the build file templates as appropriate for
each target platform.

Note that this adds extra meaning to the value of the shared_target
attribute.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7333)
diff --git a/Configurations/README b/Configurations/README
index a03d445..9fd4922 100644
--- a/Configurations/README
+++ b/Configurations/README
@@ -169,7 +169,14 @@
                            assembler files used when compiling with
                            assembler implementations.
         shared_target   => The shared library building method used.
-                           This is a target found in Makefile.shared.
+                           This serves multiple purposes:
+                           - as index for targets found in shared_info.pl.
+                           - as linker script generation selector.
+                           To serve both purposes, the index for shared_info.pl
+                           should end with '-shared', and this suffix will be
+                           removed for use as a linker script generation
+                           selector.  Note that the latter is only used if
+                           'shared_defflag' is defined.
         build_scheme    => The scheme used to build up a Makefile.
                            In its simplest form, the value is a string
                            with the name of the build scheme.