Fix build on RISC-V (#856)

Fix build on RISC-V (#856)

Compiling on RISC-V (without an explicit -latomic) fails with

    /usr/riscv64-linux-gnu/include/c++/10/ostream:611:
    undefined reference to __atomic_compare_exchange_1

Use std::atomic<uint8_t>::exchange() to detect whether -latomic
implements 1-byte compare-and-exchange API used by Squid.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>