[clang] Use 'starts_with' instead of 'startswith' in Gnu.cpp (NFC)

llvm-project/clang/lib/Driver/ToolChains/Gnu.cpp:1757:14:
 error: 'startswith' is deprecated: Use starts_with instead [-Werror,-Wdeprecated-declarations]
 1757 |     if (Flag.startswith("!march=") || Flag.startswith("-march="))
      |              ^~~~~~~~~~
      |              starts_with