tool_chain: update GCC to version 4.7.4

Fixes #1051.
dde_linux: fix bug in find_next_zero_bit_le

value '1' has a default type of int, but long is needed to cover all 64 bits
hw: clrex during context switch (Fixes #1196)
hw: fix panda instability on kernel exits

Invalidating all branch predictors before switching the PD
fixes instability problems on Panda and has not much effect
on the performance of other boards. However, we neither know why
this is a fix nor wether it fixes the real cause of the problem.

fix #1294
noux: reinitialize Genode::config
okl4: declare read-write-lock as volatile

GCC 4.7.4 and newer seems to optimize the lock-variable accesses more
radically, which uncovered the missing volatile qualifier and resulted
in:

Assertion "(int)locked >= 0" failed in file '.../okl4_x86/kernel/include/kernel/read_write_lock.h', line 151 (fn=f0104771)
--- "KD# assert" ---