lib/raid6: Let $(UNROLL) rules work with macOS userland

Older versions of BSD awk are fussy about the order of '-v' and '-f'
flags, and require a space after the flag name. This causes build
failures on platforms with an old awk, such as macOS and NetBSD.

Since GNU awk and modern versions of BSD awk (distributed with
FreeBSD/OpenBSD) are fine with either form, the definition of
'cmd_unroll' can be trivially tweaked to let the lib/raid6 Makefile
work with both old and new awk flag dialects.

Signed-off-by: John Millikin <john@john-millikin.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>