[NFCI][test] Use lit.local.cfg in test/MachineVerifier/RISCV to gate tests (#118217)

87cc4b4 added `REQUIRES:
riscv64-registered-target` to the only test in this directory. It seems
better (and also reflects what we do elsewhere, including in the AMDGPU/
subdir in MachineVerifier) to gate for all files in the directory via
lit.local.cfg.

Author: asb

llvm/test/MachineVerifier/RISCV/lit.local.cfg

@@ -0,0 +1,2 @@
+if not "RISCV" in config.root.targets:
+    config.unsupported = True

llvm/test/MachineVerifier/RISCV/subreg-liveness.mir

@@ -1,6 +1,5 @@
 # NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
 # RUN: llc -mtriple=riscv64 -mattr=+v -run-pass=none %s -o - | FileCheck %s
-# REQUIRES: riscv64-registered-target
 
 # During the MachineVerifier, it assumes that used registers have been defined
 # In this test case, while $v12_v13_v14_v15_v16 covers $v14_v15,