[XRay][test] Mark fdr-mode.cpp test as unsupported for RISC-V

Commit c6f501d fixed an issue where some tests were incorrectly
marked as unsupported for a bootstrapping build. This exposed in our
'slow' full-bootstrap qemu-system CI that the fdr-mode.cpp fails on
RISC-V. We mark it as unsupported. I believe _xray_ArgLoggerEntry needs
to be implemented in xray_trampoline_risc*.S for this to work.

Author: asb

compiler-rt/test/xray/TestCases/Posix/fdr-mode.cpp

@@ -22,6 +22,8 @@
 // RUN: rm fdr-unwrite-test-*
 // UNSUPPORTED: target=powerpc64le-{{.*}}
 /// TODO: FDR logging arg1 handler(__xray_ArgLoggerEntry) hasn't implemented yet on LoongArch
+// UNSUPPORTED: target=riscv{{.*}}
+/// TODO: FDR logging arg1 handler(__xray_ArgLoggerEntry) isn't implemented yet for RISC-V
 // UNSUPPORTED: target=loongarch64{{.*}}
 // UNSUPPORTED: armhf-linux
 // REQUIRES: built-in-llvm-tree