arch: common: use zephyr_library for all source files

The zephyr_library() used to only be called if
CONFIG_GEN_ISR_TABLES is enabled. Which means that any
zephyr_library_*() calls are putting things into another
library if CONFIG_GEN_ISR_TABLES is disabled. So pull
the call to zephyr_library() to outside. Also moves
semihost.c into the library.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>