[flang] Use llvm-readobj instead of llvm-readelf in rv64 test

In some circumstances llvm-readelf symlink to llvm-readobj appears
to not be available. For want of a proper fix in CMake, use llvm-readobj
in the test instead.

flang/test/Driver/code-gen-rv64.f90

@@ -5,11 +5,11 @@
 ! RUN: rm -f %t.o
 ! RUN: %flang_fc1 -triple riscv64-unknown-linux-gnu \
 ! RUN:   -target-feature +d -target-feature +c -emit-obj %s -o %t.o
-! RUN: llvm-readelf -h %t.o | FileCheck %s
+! RUN: llvm-readobj -h %t.o | FileCheck %s
 
 ! RUN: rm -f %t.o
 ! RUN: %flang --target=riscv64-unknown-linux-gnu -c %s -o %t.o
-! RUN: llvm-readelf -h %t.o | FileCheck %s
+! RUN: llvm-readobj -h %t.o | FileCheck %s
 
 ! If Flang failed to emit target-feature info, then Flags will be 0x0.
 ! 0x5 means set EF_RISCV_RVC (0x1) and EF_RISCV_FLOAT_ABI_DOUBLE (0x4)