Let cmake infer source file language by the file extension.

This applies default compiler flags to .S files, in particular removing
the "-pedantic" option, which is desirable because there is nothing to
reasonably warn about; and the only thing that gcc warns about is that
you allegedly can't correctly invoke GLUE2 in lib/builtins/assembly.h
on platforms for which USER_LABEL_PREFIX is the empty string.

In the gcc bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=33305 that
added the warning, a commenter notes that giving a macro of zero characters
to another macro is not precisely the same as failing to supply an argument,
and "there is a widespread belief in C++ community that such usage is valid".
Unfortunately the only way to silence the warning is to avoid -pedantic.

Differential Revision: http://reviews.llvm.org/D10713

llvm-svn: 243446