drivers: entropy: Make entropy_stm32.c clang compatible

Clang doesn't support push_options, pop_options and optimize pragmas.
Since "Ofast" is only applied to rng_pool_get() we can use function
attributes instead of pragmas.

Please note that clang doesn't support applying optimizations to
functions.

This change was verified by comparing assembler output.

Signed-off-by: Patryk Duda <pdk@semihalf.com>