lib: sbi_ecall: Split up sbi_ecall_replace

Split up sbi_ecall_replace so that each extension is in its individual
file.

Also reorganize the corresponding section in lib/sbi/objects.mk so
that it is grouped by extension, now that the object file targets are
split up.

Signed-off-by: Vivian Wang <dramforever@live.com>
Reviewed-by: Anup Patel <anup@brainfault.org>