refactor(cpufeat): use alternative encoding for "SB" barrier

The "sb" barrier instruction is a rather new addition to the AArch64
instruction set, so it is not recognised by all toolchains. On top of
that, the GNU assembler denies this instruction, unless a compatible
processor is selected:
asm_macros.S:223: Error: selected processor does not support `sb'

Provide an alternative encoding of the "sb" instruction, by using a
system register write, as this is the group where the barrier
instructions borrow their encoding space from.
This results in the exact same opcode to be generated, and any
disassembler will decode this instruction as "sb".

Change-Id: I5f44c8321e0cc04c784e02bd838e964602a96a8e
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

include/arch/aarch64/arch.h

@@ -1335,4 +1335,7 @@
 #define CPUMPMMCR_EL3_MPMM_EN_SHIFT	UINT64_C(0)
 #define CPUMPMMCR_EL3_MPMM_EN_MASK	UINT64_C(0x1)
 
+/* alternative system register encoding for the "sb" speculation barrier */
+#define SYSREG_SB			S0_3_C3_C0_7
+
 #endif /* ARCH_H */

include/arch/aarch64/asm_macros.S

@@ -214,13 +214,25 @@
 	ret
 	.endm
 
+	/*
+	 * The "sb" instruction was introduced later into the architecture,
+	 * so not all toolchains understand it. Some deny its usage unless
+	 * a supported processor is specified on the build command line.
+	 * Use sb's system register encoding to work around this, we already
+	 * guard the sb execution with a feature flag.
+	 */
+
+	.macro sb_barrier_insn
+	msr	SYSREG_SB, xzr
+	.endm
+
 	/*
 	 * Macro for using speculation barrier instruction introduced by
 	 * FEAT_SB, if it's enabled.
 	 */
 	.macro speculation_barrier
 #if ENABLE_FEAT_SB
-	sb
+	sb_barrier_insn
 #else
 	dsb	sy
 	isb
@@ -234,7 +246,7 @@
 	.macro exception_return
 	eret
 #if ENABLE_FEAT_SB
-	sb
+	sb_barrier_insn
 #else
 	dsb	nsh
 	isb