Add support for single-level GAS .rept directive. Nested .rept's are not

Add support for single-level GAS .rept directive.  Nested .rept's are not
allowed at the moment.  The implementation works mostly like a preproc; it
copies source lines and replays them to the lexer.  A new .line directive
was added to fix up line numbers for errors and warnings.

* gas-parser.h (yasm_parser_gas): Add rept structure storage.
(gas_rept): New data structure for .rept state.
(gas_rept_line): Data structure to store source lines within .rept block.

* gas-parser.c: Initialize rept to NULL and check for unclosed rept.

* gas-bison.y: Add support for .line, .rept, and .endr directives.  The
DIR_REPT handler just creates the rept structure, and the DIR_ENDR handler
just errors (.endr without .rept).  All the real work is done in the lexer.

* gas-token.re (rept_input): Replays captured .rept block source lines back
to fill().
(fill): Call rept_input() instead of yasm_preproc_input() if expanding a
rept block.
(gas_parser_lex): Capture source lines and store into rept data structures.

Also added a whole bunch of testcases.

svn path=/trunk/yasm/; revision=1306