semihosting: Move ARM semihosting code to shared directories

semihosting: Move ARM semihosting code to shared directories

This commit renames two files which provide ARM semihosting support so
that they can be shared by other architectures:

 1. target/arm/arm-semi.c     -> hw/semihosting/common-semi.c
 2. linux-user/arm/semihost.c -> linux-user/semihost.c

The build system was modified use a new config variable,
CONFIG_ARM_COMPATIBLE_SEMIHOSTING, which has been added to the ARM
softmmu and linux-user default configs. The contents of the source
files has not been changed in this patch.

Signed-off-by: Keith Packard <[email protected]>
[AJB: rename arm-compat-semi, select SEMIHOSTING]
Signed-off-by: Alex Bennée <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20210107170717.2098982[email protected]>
Message-Id: <20210108224256[email protected]>