make-asm-offsets: fix sed script

When copying the "sed" script to generate the asm-offsets.h file from
the Linux Kbuild script into the make-asm-offsets file I missed the
fact that the former runs in a "make" context and thus uses double
"$$" to escape a single "$", while the latter is a shell script, where
this must not be done.  Unfortunately the problem did not show up
during the initial tests on Power Architecture systems, but on ARM the
generated asm-offsets.h was not correct.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Tested-by: Heiko Schocher <hs@denx.de>
Tested-by: Ben Gardiner <bengardiner@nanometrics.ca>