Fix ELF64 relocations for common, global, and common+global symbols to match

Fix ELF64 relocations for common, global, and common+global symbols to match
GAS output.  The way we were generating relocations before would make
common+global symbol usage generate a relocation against the symbol but
figure in the symbol's value into the relocation addend.

* expr.h (yasm_symrec_relocate_action): New enum, so that:
(yasm_expr_extract_symrec): can conditionalize replacing the symbol with its
value based on whether the symbol is only local (e.g. not declared global,
etc).
* expr.c (yasm_expr_extract_symrec): Update implementation.

* xdf-objfmt.c, coff-objfmt.c: Update to use new enum constants.

* elf-objfmt.c (elf_objfmt_output_expr): Only relocate against section if
symbol is only local, and change call to yasm_expr_extract_symrec to only
add in symbol value if symbol is only local.

* stabs-elf.hex, elftest.hex: Update for changes.

* elf_gas64_reloc.asm: New test.

svn path=/trunk/yasm/; revision=1311