[llvm-exegesis] Require RISCV Target for new test

This test was causing buildbot failures because it requires both the
RISCV and X86 targets to be enabled to run successfully. It already
requires the X86 target by virtue of being in the X86/ test directory,
but failed to require the RISCV target, which this patch corrects.

Author: boomanaiden154

llvm/test/tools/llvm-exegesis/X86/mcpu_not_set_during_cross_compilation.s

@@ -1,3 +1,4 @@
 # RUN: not llvm-exegesis -mtriple=riscv64-unknown-linux-gnu -mode=latency --benchmark-phase=assemble-measured-code -opcode-name=ADD 2>&1 | FileCheck %s
+# REQUIRES: riscv-registered-target
 
 # CHECK: llvm-exegesis error: A CPU must be explicitly specified when cross compiling. To see all possible options for riscv64-unknown-linux-gnu triple use -mcpu=help