kernel/system_certificate.S: use real contents instead of macro GLOBAL()

If a macro is only used within 2 times, and also its contents are
within 2 lines, recommend to expand it to shrink code line.

For our case, the macro is not portable either: some architectures'
assembler may use another character to mark newline in a macro (e.g.
'`' for arc), which will cause issue.

If still want to use macro and let it portable enough, it will also
need include additional header file (e.g "#include <linux/linkage.h>",
although it also need be fixed).

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: David Howells <dhowells@redhat.com>