gas: support double-slash line comments in BPF assembly

gas: support double-slash line comments in BPF assembly

This patch makes the BPF assembler to support double-slash line
comments, like the llvm BPF assembler does.  At this point both
assemblers support the same commenting styles:

- Line comments preceded by # or //.
- Non-nestable block comments delimited by /* and */.

This patch also adds a couple of tests to make sure all the comment
styles work in both normal and pseudoc syntax.  The manual is also
updated to mention double-slash line comments.