Add a new program (link_tool_exe.c) and use this to link the tool

Add a new program (link_tool_exe.c) and use this to link the tool
executables.  Gets rid of the linker script kludgery and uniformly
uses -Ttext=0x38000000 (or whatever) on Linux, so as to accomodate
both traditional ld and gold.  Should fix #193413 although I have
been unable to test it.  Using a whole new program seems like
overkill, but this is infrastructure to support static linking of
the tool executables on MacOS too.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@11141