x86-64: Hide internal assembly functions

x86-64: Hide internal assembly functions

Hide x86-64 internal assembly functions. Before

$ nm -D lib/libzstd.so.1 | grep usingDTable_internal_bmi2_asm_loop
00000000000c23c0 T _HUF_decompress4X1_usingDTable_internal_bmi2_asm_loop
00000000000c23c0 T HUF_decompress4X1_usingDTable_internal_bmi2_asm_loop
00000000000c283d T _HUF_decompress4X2_usingDTable_internal_bmi2_asm_loop
00000000000c283d T HUF_decompress4X2_usingDTable_internal_bmi2_asm_loop
$

After

$ nm -D lib/libzstd.so.1 | grep usingDTable_internal_bmi2_asm_loop
$

This fixes issue #2990.