[X86] Add ISel patterns to select 'f32_to_f16' and 'f16_to_f32' dag nodes.

This patch adds tablegen patterns to select F16C float-to-half-float
conversion instructions from 'f32_to_f16' and 'f16_to_f32' dag nodes.

If the target doesn't have F16C, then 'f32_to_f16' and 'f16_to_f32'
are expanded into library calls.
Add the -just-symbol-name (aka -j) flag to llvm-nm to just print the
symbol’s name.  On darwin the -j flag is used (often in combinations
with other flags) to produce a complete list of symbol names which
than can then be reorder and used with ld(1)’s -order_file.