BUILD: makefile: fix regex syntax in ARM platform detection

Commit d93e6ec ("BUILD: on ARM, must be linked to libatomic.") broke the
build due to a missing backslash in front of the '#':

  Makefile:331: *** invalid syntax in conditional.  Stop.

Let's address this and make sure we only pick relevant lines (and not
possibly empty lines).

(cherry picked from commit 67b095e797a156ae27b7b52f6ccf57171717dd16)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit ff97b2fbcf5e7f33231c2ddaa8e0037af5977121)
Signed-off-by: Willy Tarreau <w@1wt.eu>
1 file changed