Add the lexing code for the new portable assembler.
It is internal to the assembler, so lives in a subdirectory of cmd/asm/internal.
Its only new dependency is the flags package for the assembler, so
add that too; it's trivial. That package manages the command-line
flags in a central place.
The lexer builds on text/scanner to lex the input, including doing a
Plan 9-level implementation of the C preprocessor.