[OpenMP] Only test kmp_atomic_float10_max_min.c on X86

The test `openmp/runtime/test/atomic/kmp_atomic_float10_max_min.c` uses a compiler
flag `-mlong-double-80` that might not be supported by all targets. Currently it
requires `x86-registered-target`, but that requirement can be true when LLVM
supports X86 while the actual `libomp` arch is not X86. For example, when LLVM
is built on AArch64 with all targets enabled, `x86-registered-target` can be met.
If `libomp` is built with native target, aka. AArch64, the test will still be enabled,
causing test failure.

This patch only enables the test if the actual target is X86. The actual target
is determined by `LIBOMP_ARCH`.

Fix #53696.

Reviewed By: jlpeyton

Differential Revision: https://reviews.llvm.org/D142172