semihosting: reorganize semihosting commands

semihosting: reorganize semihosting commands

the same semihosting handlers chain is declared twice:
 1. in src/target/armv4_5.c
 2. in src/target/riscv/riscv.c

to make it simpler we moved the declaration into
'src/target/semihosting_common.c' under semihosting_common_handlers[].
then we used this into both of armv4_5.c and riscv.c

Change-Id: If813b3fd5eb2476658f1308f741c4e805141f617
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5473
Tested-by: jenkins
Reviewed-by: Muhammad Omair Javaid <omair.javaid@linaro.org>
Reviewed-by: Tim Newsome <tim@sifive.com>
Reviewed-by: Liviu Ionescu <ilg@livius.net>