[libc] Move statement inside #ifdef/#endif to prevent unused variable warning

This is a fix forward for arm32
`/llvm/libc_worker/worker/libc-arm32-debian/libc-arm32-debian-dbg/llvm-project/libc/src/__support/big_int.h:259:20: error: unused variable 'TOTAL_BITS' [-Werror,-Wunused-variable]`

https://lab.llvm.org/buildbot/#/builders/229/builds/24791