lib: Add common semihosting library

We factor out the arch-independent parts of the ARM semihosting
implementation as a common library so that it can be shared
with RISC-V.

Signed-off-by: Kautuk Consul <kconsul@ventanamicro.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/lib/Makefile b/lib/Makefile
index f2cfd1e..d77b33e 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -146,6 +146,8 @@
 obj-y += rtc-lib.o
 obj-$(CONFIG_LIB_ELF) += elf.o
 
+obj-$(CONFIG_$(SPL_TPL_)SEMIHOSTING) += semihosting.o
+
 #
 # Build a fast OID lookup registry from include/linux/oid_registry.h
 #