Disable --generate-debug-info by default in all cases.

The option is disabled by default unless explicitly enabled.
In particular, it is no longer enabled in debug builds,
and the --debuggable option does not affect it.

I want to use this flag to control the debug data generated
by the JIT as well.  Since this takes run-time memory,
I want to avoid enabling it unless explicitly requested.

Change-Id: I2e2afa2f56bb0a113e92cc2e26e00dceac1689ca