Re-apply "[ORC][ORC-RT] Add initial native-TLV support to MachOPlatform."

Reapplies fe1fa43f, which was reverted in
6d8c6394, with fixes:

1. Remove .subsections_via_symbols directive from macho_tlv.x86-64.s (it's
not needed here anyway).

2. Return error from pthread_key_create to the MachOPlatform to silence unused
variable warning.