compiler-gcc: remove comment about gcc 4.5 from unreachable()

compiler-gcc: remove comment about gcc 4.5 from unreachable()

Remove the comment about being unable to detect __builtin_unreachable.
__builtin_unreachable was implemented in the GCC 4.5 timeframe. The
kernel's minimum supported version of GCC is 4.6 since commit
cafa0010cd51 ("Raise the minimum required gcc version to 4.6"). Commit
cb984d101b30 ("compiler-gcc: integrate the various compiler-gcc[345].h
files") shows that unreachable() had different guards based on GCC
version.

Suggested-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>