[lld][macho][NFC] Make MachO/start-end.s test less britle by checking for _main:

In start-end.s there is a lit check line `# SEG: _main` to begin the
check at the start of the function main where `_main` is the Darwin name
mangling for C main. Because the text file that FileCheck is getting as
input has the path of the compiler build in it from llvm-mc and
llvm-objdump, and because of the lack of a trailing colon in this check
line we end up inadvertently matching against the line of text with the
compiler path in it in the case where said path contains "_main" some
place. This can be very likely if the compiler branch has "main" or
"_main" in it.

To fix this I include the training : since that will match on the
function label and not the path line.