gccrs: lex: Prevent directories in RAIIFile

gccrs: lex: Prevent directories in RAIIFile

RAIIFile constructor was accepting directory filename. This lead to
unattended directory opening in some part of the code (load_file_bytes)
wich resulted in ice. Since RAIIFile are used for the lexer, removing
the ability to open directories with RAIIFile fixes those issues and
prevent future mistakes.

gcc/rust/ChangeLog:

	* lex/rust-lex.h: Add file type check.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>