arm: add support for semihosting for ARMv7M targets

arm: add support for semihosting for ARMv7M targets

If you enable CONFIG_SEMIHOSTING for STM32F429 target, you will get compile
error looking like this:

arch/arm/lib/semihosting.c: In function 'smh_read':
{standard input}: Assembler messages:
{standard input}:34: Error: invalid swi expression
{standard input}:34: Error: value of 1193046 too large for field of 2 bytes at 0
scripts/Makefile.build:277: recipe for target 'arch/arm/lib/semihosting.o' failed

The source of the problem is "svc #0x123456" instruction. This instruction
can not be encoded using Thumb2 instruction set used by ARMv7M CPUs.
ARM documentation suggests using "bkpt #0xAB" instruction instead [1].
This patch fixes compile errors and adds support for semihosting for
STM32F429 or any other ARMv7M target.
This change was sested on STM32F429-DISCOVERY board using OpenOCD and
"smhload" u-boot command.

[1] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/Bgbjhiea.html

Signed-off-by: Vadzim Dambrouski <pftbest@gmail.com>