build/core: Remove useless warning when using OpenJDK

We provide and use a custom prebuilt OpenJDK 7 by default, so remove
this useless warning.

Change-Id: I61578c6b966bc72b328404c4a74e585a4f781d93
build/core: Disable aggressive loop optimizations

GCC 4.8 enables aggressive loop optimizations, which seems to break
NFC on some devices (possibly other things?), so lets disable it.

Also revert 2 optimizations that could possibly introduce new issues,
these are fairly untested and stability is key. We will revisit these
at a later time once fully benchmarked and tested.

Change-Id: I266dc0e4ae7894d5492b847864c35514c04a80da
envsetup: Add an option to select host gcc version

To use 4.8, you must install GCC 4.8 on the host machine.

Tested:
Running LinuxMint 15 (Raring 13.04) with host gcc 4.8 from
https://launchpad.net/~ubuntu-toolchain-r/+archive/test

Usage:
Type "hostgcc". 1=4.7 and 2=4.8.

Change-Id: I2a53a98c7bae03f1e23abf6a474a1785c3c11f5d