Add a build config for bootstrapping at -Og

[Jeff Law]

On 7/7/19 3:40 AM, Richard Sandiford wrote:
> Although BOOT_CFLAGS can be used to bootstrap with -Og, having a
> dedicated build config is sometimes more convenient.
> 
> Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?
> 
> Richard
> 
> 
> 2019-07-07  Richard Sandiford  <richard.sandiford@arm.com>
> 
> config/
> 	* bootstrap-Og.mk: New file.
> 
> gcc/
> 	* doc/install.texi (bootstrap-Og): Document.
OK
jeff