[llvm-objdump] Make arm64ec.yaml warning check less strict to account for Windows

When 7a28b0b added this it wanted
"llvm-objdump: warning: <...>". On our Windows on Arm bot you get:
c:\users\tcwg\llvm-worker\clang-arm64-windows-msvc\stage1\bin\llvm-objdump.exe: warning:

Seems unlikely the warning would come from anywhere else and if
it does, this test isn't the one to be catching that. So I've
shortened the checks to start at "warning: ".

Author: DavidSpickett

llvm/test/tools/llvm-objdump/COFF/arm64ec.yaml

@@ -27,8 +27,8 @@
 # ARM64EC: file format coff-arm64ec
 # ARM64X:  file format coff-arm64x
 
-# WARN1: llvm-objdump: warning: '{{.*}}': unable to get target for 'x86_64--', see --version and --triple.
-# WARN2: llvm-objdump: warning: '{{.*}}': unable to get target for 'x86_64-w64-windows-gnu', see --version and --triple.
+# WARN1: warning: '{{.*}}': unable to get target for 'x86_64--', see --version and --triple.
+# WARN2: warning: '{{.*}}': unable to get target for 'x86_64-w64-windows-gnu', see --version and --triple.
 
 # DISASM:      Disassembly of section .text:
 # DISASM-EMPTY: