Some ArmPL vector math routines that operate on fixed-width
vector types obey the aarch64_vector_pcs calling convention,
e.g. armpl_vsinq_f64, etc. This patch adds support for
functions in the TLI to specify an optional calling
convention to be added to the declared function. Once this
patch lands I intend to follow up with a vectoriser patch
that adds this optional calling convention when invoking the
vector library variant from the loop vectoriser.
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