kbuild: consolidate redundant sed script ASM offset generation

This part ended up in redundant code after touched by multiple
people.

[1] Commit 3234282f ("x86, asm: Fix CFI macro invocations to
deal with shortcomings in gas") added parentheses for defined
expressions to support old gas for x86.

[2] Commit a22dcdb0 ("x86, asm: Fix ancient-GAS workaround")
split the pattern into two to avoid parentheses for non-numeric
expressions.

[3] Commit 95a2f6f7 ("Partially revert patch that encloses
asm-offset.h numbers in brackets") removed parentheses from numeric
expressions as well because parentheses in MN10300 assembly have a
special meaning (pointer access).

Apparently, there is a conflict between [1] and [3].  After all,
[3] took precedence, and a long time has passed since then.

Now, merge the two patterns again because the first one is covered
by the other.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>