configure: Don't use the __atomic_*_16 functions for testing 128-bit support

configure: Don't use the __atomic_*_16 functions for testing 128-bit support

The test for 128-bit atomics is causing trouble with FreeBSD 12.2 and
--enable-werror:

 cc -Werror -fPIE -DPIE -std=gnu99 -Wall -m64 -mcx16 -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -Wstrict-prototypes -Wredundant-decls -Wundef -Wwrite-strings -Wmissing-prototypes -fno-strict-aliasing -fno-common -fwrapv -Wold-style-definition -Wtype-limits -Wformat-security -Wformat-y2k -Winit-self -Wignored-qualifiers -Wempty-body -Wnested-externs -Wendif-labels -Wexpansion-to-defined -Wno-initializer-overrides -Wno-missing-include-dirs -Wno-shift-negative-value -Wno-string-plus-int -Wno-typedef-redefinition -Wno-tautological-type-limit-compare -fstack-protector-strong -o config-temp/qemu-conf.exe config-temp/qemu-conf.c -pie -Wl,-z,relro -Wl,-z,now -m64 -fstack-protector-strong
 config-temp/qemu-conf.c:4:7: error: implicit declaration of function '__atomic_load_16' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
   y = __atomic_load_16(&x, 0);
       ^
 config-temp/qemu-conf.c:5:3: error: implicit declaration of function '__atomic_store_16' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
   __atomic_store_16(&x, y, 0);
   ^
 config-temp/qemu-conf.c:5:3: note: did you mean '__atomic_load_16'?
 config-temp/qemu-conf.c:4:7: note: '__atomic_load_16' declared here
   y = __atomic_load_16(&x, 0);
       ^
 config-temp/qemu-conf.c:6:3: error: implicit declaration of function '__atomic_compare_exchange_16' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
   __atomic_compare_exchange_16(&x, &y, x, 0, 0, 0);
   ^
 3 errors generated.

Looking for they way we are using atomic functions in QEMU, we are not
using these functions with the _16 suffix anyway. Switch to the same
functions that we use in the include/qemu/atomic.h header.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210317110512.583747-2-thuth@redhat.com>
Message-Id: <20210323165308.15244-12-alex.bennee@linaro.org>