Add TARGET_GCC_VERSION

Default to 4.6, but can overwritten to something else in
prebuilts/gcc/$os/$arch/$arch-linux-android*-$ver

Change-Id: I16f99d642b8ff8e831d392261a0c536041107051
[ARM] Extend the usage of TARGET_GCC_VERSION for gdb and arm-eabi-*.

In the future to upgrade the gcc version we just need to edit one
line in core/config.mk.

Change-Id: I01bb38c38f2388913bb7369aa4b2da480411dfbe
Export TARGET_GCC_VERSION as an env variable.

Have it referenced by the adbs tool to pick the specified toolchain.

Change-Id: I23df88946254a55d0749d6753e54559faf33e121
x86: enable FORTIFY_SOURCE

Enable FORTIFY_SOURCE for Android x86.  This adds certain security
protections to Android programs, checking for common programming
mistakes and buffer overflows.

FORTIFY_SOURCE protections have been enabled for ARM since
a46c480e43cc61d0190569684f0a1e53a5da75b6 (Jun 4th 2012)

Change-Id: Idae6f93d37ddae605088867cd3029f12632456e1