lib: Add a fifo implementation.

Implement a fifo to accomodate outstanding IPIs for a specific hart
at the same time.

Signed-off-by: Atish Patra <atish.patra@wdc.com>

lib/objects.mk

@@ -15,6 +15,7 @@ lib-objs-y += riscv_locks.o
 lib-objs-y += sbi_console.o
 lib-objs-y += sbi_ecall.o
 lib-objs-y += sbi_emulate_csr.o
+lib-objs-y += sbi_fifo.o
 lib-objs-y += sbi_hart.o
 lib-objs-y += sbi_illegal_insn.o
 lib-objs-y += sbi_init.o
@@ -23,3 +24,6 @@ lib-objs-y += sbi_misaligned_ldst.o
 lib-objs-y += sbi_system.o
 lib-objs-y += sbi_timer.o
 lib-objs-y += sbi_trap.o
+
+# External Libraries to include
+PLATFORM_INCLUDE_LIBC=y