arch-arm: HLT using immediate when checking for semihosting

HLT can use the immediate field when checking for semihosting,
rather than re-parsing it from the machInst variable.

Change-Id: I072cb100029da34d129b90c5d17e1728f9016c88
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8369
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

src/arch/arm/isa/insts/misc64.isa

@@ -177,7 +177,7 @@ let {{
 
     hltCode = '''
     ThreadContext *tc = xc->tcBase();
-    if (ArmSystem::haveSemihosting(tc) && bits(machInst, 20, 5) == 0xF000) {
+    if (ArmSystem::haveSemihosting(tc) && imm == 0xF000) {
         X0 = ArmSystem::callSemihosting64(tc, X0 & mask(32), X1);
     } else {
         // HLT instructions aren't implemented, so treat them as undefined