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

When 7a28b0b6 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: ".