[NFC] Enable atomic tests on AIX

These tests pass on AIX.

Author: jakeegan

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/fetch_add.pass.cpp

@@ -6,7 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 // UNSUPPORTED: c++03, c++11, c++14, c++17
-// UNSUPPORTED: LIBCXX-AIX-FIXME
 // XFAIL: !has-64-bit-atomics
 
 // https://github.com/llvm/llvm-project/issues/72893

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/fetch_sub.pass.cpp

@@ -6,7 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 // UNSUPPORTED: c++03, c++11, c++14, c++17
-// UNSUPPORTED: LIBCXX-AIX-FIXME
 // XFAIL: !has-64-bit-atomics
 
 // https://github.com/llvm/llvm-project/issues/72893

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.minus_equals.pass.cpp

@@ -6,7 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 // UNSUPPORTED: c++03, c++11, c++14, c++17
-// UNSUPPORTED: LIBCXX-AIX-FIXME
 // XFAIL: !has-64-bit-atomics
 
 // floating-point-type operator-=(floating-point-type) volatile noexcept;

libcxx/test/std/atomics/atomics.types.generic/atomics.types.float/operator.plus_equals.pass.cpp

@@ -6,7 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 // UNSUPPORTED: c++03, c++11, c++14, c++17
-// UNSUPPORTED: LIBCXX-AIX-FIXME
 // XFAIL: !has-64-bit-atomics
 
 // floating-point-type operator+=(floating-point-type) volatile noexcept;