cmd: add exception command

The 'exception' command allows to test exception handling.

This implementation supports ARM, x86, RISC-V and the following exceptions:
* 'breakpoint' - prefetch abort exception (ARM 32bit only)
* 'unaligned'  - data abort exception (ARM only)
* 'undefined'  - undefined instruction exception

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/cmd/Makefile b/cmd/Makefile
index 6b1c6b0..7864fcf 100644
--- a/cmd/Makefile
+++ b/cmd/Makefile
@@ -173,6 +173,8 @@
 # Android Verified Boot 2.0
 obj-$(CONFIG_CMD_AVB) += avb.o
 
+obj-$(CONFIG_ARM) += arm/
+obj-$(CONFIG_RISCV) += riscv/
 obj-$(CONFIG_X86) += x86/
 
 obj-$(CONFIG_ARCH_MVEBU) += mvebu/