AArch32: Add support for HLT to Mixed Mode models

The Semihosting v2 protocol requires us to output the Armv8-a HLT instruction
when in mixed mode (SEMIHOST_V2_MIXED_MODE), however it also requires this to
be done for Armv7-a and earlier architectures.

The HLT instruction is defined in the undefined encoding space for older
architectures but simulators such as QEMU already trap on it [1] for all
architectures and is a requirement for semihosting v2 [2].

Unfortunately the GAS restricts the use of HLT to Armv8-a which requires us to
use the instruction encodings we want directly in crt0.

This patch does this, I have not updated newlib/libc/* as that is quite out of
date already.  A proper sync is needed in order to get things back in sync.

A different patch for this would be best.

[1] 19a6e31c9d
[2] https://developer.arm.com/docs/100863/latest/the-semihosting-interface

libgloss/arm/crt0.S

@@ -116,10 +116,10 @@
 	bkpt	AngelSWI
 #elif defined(__thumb2__)
 	/*  We are in thumb mode for startup on armv7 architectures. */
-	AngelSWIAsm	AngelSWI
+	AngelSWIAsm (AngelSWI)
 #else
 	/*  We are always in ARM mode for startup on pre armv7 archs. */
-	AngelSWIAsm	AngelSWI_ARM
+	AngelSWIAsm (AngelSWI_ARM)
 #endif
 	ldr	r0, .LC0	/*  point at values read */
 
@@ -297,7 +297,7 @@ __change_mode:
 #else
 	movs	r0, #AngelSWI_Reason_GetCmdLine
 	ldr	r1, .LC30	/*  Space for command line */
-	AngelSWIAsm	AngelSWI
+	AngelSWIAsm (AngelSWI)
 	ldr	r1, .LC30
 	ldr	r1, [r1]
 #endif