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/MAINTAINERS b/MAINTAINERS
index aa4b3bc..b9cb686 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -86,6 +86,7 @@
 S:	Maintained
 T:	git git://git.denx.de/u-boot-arm.git
 F:	arch/arm/
+F:	cmd/arm/
 
 ARM ALTERA SOCFPGA
 M:	Marek Vasut <marex@denx.de>
@@ -677,6 +678,7 @@
 S:	Maintained
 T:	git git://git.denx.de/u-boot-riscv.git
 F:	arch/riscv/
+F:	cmd/riscv/
 F:	tools/prelink-riscv.c
 
 ROCKUSB
@@ -788,6 +790,7 @@
 S:	Maintained
 T:	git git://git.denx.de/u-boot-x86.git
 F:	arch/x86/
+F:	cmd/x86/
 
 XTENSA
 M:	Max Filippov <jcmvbkbc@gmail.com>