tools/Makefile: Drop the use of $(file ...)

tools/Makefile: Drop the use of $(file ...)

It is only available in make 4.0 and later, and not for example in CentOS 7.

Rewrite the logic to use echo and shell redirection, using a single capture
group to avoid having 12 different processes in quick succession each
appending one line to the file.

Fixes: 52dbd6f07cea7a ("tools: generate pkg-config files from make variables")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wl@xen.org>