At least one bin-utils version for ARM is know to ignore undefined
symbols when performing the final link of vmlinux.
Add an explicit check for undefined symbols to catch this.
The check is made in combination with generating the System.map file
and the actual algorithm is moved to a small shell script - mksysmap.

External symbols with three leading underscores are ignored - sparc
uses them for the BTFIXUP logic.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>