[libc] Temporarily disable arm32's sinf, cosf, sincosf entrypoints.

With correctly rounded implementations, these functions will be tested for all
rounding modes.  Since fegetround and fesetround are not implemented for arm32,
these tests will fail in all non-default rounding modes.  We will re-enable
these entrypoints and tests once fegetround and fesetround are implemented for
arm32.