locking/atomic: simplify ifdef generation

In gen-atomic-fallback.sh's gen_proto_order_variants(), we generate some
ifdeferry with:

| local basename="${arch}${atomic}_${pfx}${name}${sfx}"
| ...
| printf "#ifdef ${basename}\n"
| ...
| printf "#endif /* ${arch}${atomic}_${pfx}${name}${sfx} */\n\n"

For clarity, use ${basename} for both sides, rather than open-coding the
string generation.

There is no change to any of the generated headers as a result of this
patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210713105253.7615-2-mark.rutland@arm.com