tls_index structure as part of an adrp instruction using a 21 bit PC
relative value. Used in conjunction with
BFD_RELOC_AARCH64_TLSGD_ADD_LO12_NC.
+ENUM
+ BFD_RELOC_AARCH64_TLSGD_ADR_PREL21
+ENUMDOC
+ AArch64 TLS General Dynamic
ENUM
BFD_RELOC_AARCH64_TLSGD_ADD_LO12_NC
ENUMDOC