Feature flag handling was not perfect, +nofp16 disabled fp
instructions too.
New feature flag macros were added to check features with multiple
bits set (matters for FP_F16 and SIMD_F16 opcode feature tests).
The unused AARCH64_OPCODE_HAS_FEATURE was removed, all checks should
use one of the AARCH64_CPU_HAS_* macros. AARCH64_CPU_HAS_FEATURE
now checks all feature bits.
The aarch64_features table now contains the dependencies as
a separate field (so when the feature is enabled all dependencies
are enabled and when it is disabled everything that depends on it
is disabled).
Note that armv8-a+foo+nofoo is not equivalent to armv8-a if
+foo turns on dependent features that nofoo does not turn off.