ARM: use arch_extension directive instead of arch argument

The LLVM Target parser currently does not allow to specify the security
extension as part of -march (see also LLVM Bug 40186 [0]). When trying
to use Clang with LLVM's integrated assembler, this leads to build
errors such as this:
  clang-8: error: the clang compiler does not support '-Wa,-march=armv7-a+sec'

Use ".arch_extension sec" to enable the security extension in a more
portable fasion. Also make sure to use ".arch armv7-a" in case a v6/v7
multi-platform kernel is being built.

Note that this is technically not exactly the same as the old code
checked for availabilty of the security extension by calling as-instr.
However, there are already other sites which use ".arch_extension sec"
unconditionally, hence de-facto we need an assembler capable of
".arch_extension sec" already today (arch/arm/mm/proc-v7.S). The
arch extension "sec" is available since binutils 2.21 according to
its documentation [1].

[0] https://bugs.llvm.org/show_bug.cgi?id=40186
[1] https://sourceware.org/binutils/docs-2.21/as/ARM-Options.html

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Mans Rullgard <mans@mansr.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Olof Johansson <olof@lixom.net>

arch/arm/mach-keystone/Makefile

@@ -1,9 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 obj-y					:= keystone.o smc.o
 
-plus_sec := $(call as-instr,.arch_extension sec,+sec)
-AFLAGS_smc.o				:=-Wa,-march=armv7-a$(plus_sec)
-
 obj-$(CONFIG_SMP)			+= platsmp.o
 
 # PM domain driver for Keystone SOCs

arch/arm/mach-keystone/smc.S

@@ -21,6 +21,7 @@
  *
  * Return: Non zero value on failure
  */
+	.arch_extension sec
 ENTRY(keystone_cpu_smc)
 	stmfd   sp!, {r4-r11, lr}
 	smc	#0