[flang] AArch64: Set flushing mode for subnormals on glibc and bionic based systems

Signed-off-by: Paul Osmialowski <pawel.osmialowski@arm.com>

Original-commit: flang-compiler/f18@4881e3a91c34671a38ef410a771cd8e0e7a62e10
Reviewed-on: https://github.com/flang-compiler/f18/pull/397