OMAP4: enable smc instruction in new assembler versions

New assemblers need -march=armv7-a+sec on command line or
.arch_extension sec inline to enable use of the smc instruction.

This patch uses as-instr to check the latter to conditionally
enable the former in AFLAGS for files that use smc.

Checked on both old and new binutils to verify that it does
not break old versions.

Signed-off-by: John Rigby <john.rigby@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>