[XRay][test] Remove unneeded REQUIRES: x86_64-target-arch

fdr-thread-order.cpp can be very slow when the thread contention is large.
Enable it for AArch64 and x86-64 for now.

fdr-mode.cpp fails on a ppc64le machine. Unsupport it on ppc64le for now.

The remaining modified tests pass on AArch64, ppc64le, and x86-64.