COFF: Infer machine type earlier than before.

Previously, we infer machine type at the very end of linking after
all symbols are resolved. That's actually too late because machine
type affects how we mangle symbols (whether or not we need to
add "_").

For example, /entry:foo adds "_foo" to the symbol table if x86 but
"foo" if x64.

This patch moves the code to infer machine type, so that machine
type is inferred based on input files given via the command line
(but not based on .directives files).

llvm-svn: 241843