Add a build config for bootstrapping at -Og

Although BOOT_CFLAGS can be used to bootstrap with -Og, having a
dedicated build config is sometimes more convenient.

2019-07-08  Richard Sandiford  <richard.sandiford@arm.com>

config/
	* bootstrap-Og.mk: New file.

gcc/
	* doc/install.texi (bootstrap-Og): Document.

From-SVN: r273193