Upstream: GCC PGO and LTO support

One result of this project has been upstreamed PGO support in the GCC ebuilds. It is highly recommended that you compile GCC with PGO, as it really helps with compile times. Simply add pgo to your sys-devel/gcc USE flags and emerge and you're all set. The initial GCC compilation time will increase, however all subsequent compilations will be much faster. As of May 2019, GCC may be built using LTO and PGO in conjunction -- add lto to your USE when emerging gcc. If you want to enable all optimizations that you normally would use, add CFLAGS to BOOT_CFLAGS when emerging. This may become the default in the future.