Fix restore_rt on PowerPC

[taylordotfish]

On PowerPC, Clang fails to compile code generated by the C backend, due to the implementation of restore_rt:

$ cat main.zig
pub fn main() void {}
$ zig build-exe -ofmt=c -OReleaseSmall main.zig
$ clang -I/opt/zig/lib -nostartfiles main.c
main.c:1488:2: error: non-ASM statement in naked function is not supported
 1488 |  register uintptr_t const t0 __asm("r0") = (uintptr_t)172ul;
      |  ^
main.c:1487:8: note: attribute is here
 1487 | static zig_naked zig_noreturn void os_linux_powerpc64_restore_rt__3126(void) {
      |        ^

This is because restore_rt on PowerPC uses a register input operand for the syscall number, which is compiled as a separate register uintptr_t declaration, disallowed in naked functions.

Following the same approach as x86 and ARM, when using the C backend, this PR employs alternative inline assembly that avoids using non-immediate input operands.

This PR also adds some registers that were missing from the list of clobbers. As with all syscalls, the registers r0, r3, ctr, and xer should also be specified as clobbers or outputs (see musl).