[backlog] Automatically detect ARM instructionset on GNU

ARMv6 should be auto detected. Currently arm/armv6 can be passed as -embedded options. The benefit of ARMv6 in src/corelib is the use of atomic load and exchange, instead of the old "swap" extension. _ARCH_ARM_6_ should be used to detect it. The first part of ARM detection was done.

http://qt.gitorious.org/~zecke/qt/zeckes-qt/commit/50b67cf1f47d2caa5b876175129ff8943cba616a