gcc: Provide _sbrk implementation compatible with RTX

I verified that the hang issue I was seeing when building and running
the mbed official networking tests with GCC_ARM was related to this
issue reported on the mbed forums:
    http://mbed.org/forum/mbed/topic/3803/?page=1#comment-18934

If you are using the 4.7 2013q1 update of GCC_ARM or newer then it
will have a _sbrk() implementation which checks the new top of heap
pointer against the current thread SP, stack pointer.
See this GCC_ARM related thread for more information:
    https://answers.launchpad.net/gcc-arm-embedded/+question/218972
When using RTX RTOS threads, the thread's stack pointer can easily
point to an address which is below the current top of heap so this
check will incorrectly fail the allocation.

I have added a _sbrk() implementation to the mbed SDK which checks the
heap pointer against the MSP instead of the current thread SP.  I have
only enabled this for TOOLCHAIN_GCC_ARM as this is the only GCC based
toolchain that I am sure requires this.