Minor additions to instruction parser in text-based specification.
Minor additions to instruction parser in text-based specification.

Specifically:
 - support negative numbers
 - support indirect jump targets
 - support comments
 - fix problem with regex short sightedness

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3453
TEST= ./scons run_spec_val_test
TEST= ./scons dfacheckvalidator
R=halyavin@google.com

Review URL: https://codereview.chromium.org/17247003

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@11560 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client