win: Omit ".exe" from lld warning and error messages.

This is a minor follow-up to https://reviews.llvm.org/D49189. On Windows, lld
used to print "lld-link.exe: error: ...". Now it just prints "lld-link: error:
...". This matches what link.exe does (it prints "LINK : ...") and makes lld's
output less dependent on the host system.

https://reviews.llvm.org/D51133

llvm-svn: 340487