[PATCH 0/2] libm: Changes to errno and math_errhandling

Here are a couple of related patches that adjust errno and exception
configuration stuff.

The first patch makes libm/common use the same configuration value as
libm/math to decide whether to set errno when exceptions occur. With
that, users of libm/math get consistent errno reporting across the
whole math library when using the normal API. When errno is enabled,
calls to some __ieee754 symbols may end up setting errno still.

The second patch sets the math_errhandling value based on library and
hardware configuration so that applications can tell whether
exceptions and errno are supported. This requires configuration on a
per-target basis in machine/ieeefp.h; I've added configuration for
arm, aarch64, x86, x86_64 and RISC-V. I think this could be automated.

Reporting on whether exceptions are supported suffers from
inconsistent exception support for hardware that uses a mixture of
hardware and software for floating point operations. I haven't found
any standards-conformant mechanism to use though.