package/systemd: Add support of ARC64 architecture

[vkremneva]

When building systemd-249.1 for ARC64 with Buildroot, getting following error:

----------------------------------------------------8<----------------------------------------------------
In file included from ../src/basic/architecture.c:5:
../src/basic/architecture.h:233:4: error: #error "Please register your architecture here!"
233 | # error "Please register your architecture here!"
| ^~~~~
----------------------------------------------------8<----------------------------------------------------

Mention of arc in architecture.c file: https://github.com/systemd/systemd/blob/main/src/basic/architecture.c#L118
How arc support was added: systemd/systemd@27b09f1

From @abrodkin:

After systemd/systemd@35b42e5#diff-0da14956e96dad3a49c7cb88f2eb1a086c7198323dd8f2c5a0d2591ef92c9f39 changes should be performed here: https://github.com/systemd/systemd/blob/main/src/basic/missing_syscalls.py