Update to GCC 12 for development branch (RTEMS 6)

[Sebastian Huber]

Hello,

I updated the RSB to use the latest GCC 12 release branch version. GCC 
12.1 was released on May 6th:

https://gcc.gnu.org/gcc-12/changes.html

One interesting GCC 12 feature for embedded systems is the default 
initialization of stack variables:

https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Optimize-Options.html#index-ftrivial-auto-var-init

I added a back port for improved gcov support for which I missed the GCC 
12 development stage:

https://gcc.gnu.org/onlinedocs/gcc/Freestanding-Environments.html#Freestanding-Environments

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