scripts/mksysmap: Fix badly escaped '$'

The backslash characters escaping '$' in the command to sed (intended to
prevent it from interpreting '$' as "end-of-line") are currently being
consumed by the Shell (where they mean that sh should not evaluate what
follows '$' as a variable name). This means that

    sed -e "/ \$/d"

executes the script

    / $/d

instead of the intended

    / \$/d

So escape twice in mksysmap any '$' that actually needs to reach sed
escaped so that the backslash survives the Shell.

Fixes: c4802044 ("scripts/mksysmap: use sed with in-line comments")
Fixes: 320e7c9d

 ("scripts/kallsyms: move compiler-generated symbol patterns to mksysmap")
Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>