core/cortex-m: mark gcc helper function "used"
gcc injects calls to that function without notifying LTO.
BRANCH=cr50, cr50-mp
TEST=after the entire stack was applied Cr50 builds successfully with
CONFIG_LTO enabled, and passes TCG test suite.
@@ -14,7 +14,7 @@ union words {
uint32_t w[2];
};
-uint64_t __aeabi_llsr(uint64_t v, uint32_t shift)
+uint64_t __attribute__((used)) __aeabi_llsr(uint64_t v, uint32_t shift)
{
union words val;
union words res;
