start a linker script helper file

Start a common header file for common linker script code (such as
workarounds for older linkers) rather than doing this in the build system.

As fallout, we no longer execute the linker every time config.mk is
included by a build file (which can easily be 70+ times), but rather only
execute it once.

This also fixes a bug in the major version checking by creating a macro to
easily compare versions and keep people from making the same common
mistake (forgetting to check major and minor together).

Signed-off-by: Mike Frysinger <vapier@gentoo.org>