[aaelf64] No static TLS 64-bit relocation with expression DTPREL(S+A)

[smithp35]

Both clang and GCC leave out the DW_AT_location for TLS variables ostensibly due to lack of a relocation to describe the location in the debug data.

For x86_64 DW_AT_location is emitted as: DW_AT_location (DW_OP_const8u 0x0, DW_OP_form_tls_address) With the relocation: R_X86_64_DTPOFF64 to the symbol definition.

In AArch64 the closest equivalent is R_AArch64_TLS_DTPREL although it is marked as a dynamic relocation (normally used for the GOT entry created by the global-dynamic in the traditional dialect). If we marked this relocation as static and dynamic (and made sure that static linkers could handle it in a static context), then we could support DW_AT_location

LLVM review that removed the DW_AT_location https://reviews.llvm.org/D43860

For example:

__thread int foo;

int main(void) {
  return foo;
}

With clang -g -O2 --target=aarch64-linux-gnu

0x00000023:   DW_TAG_variable
                DW_AT_name      ("foo")
                DW_AT_type      (0x0000002b "int")
                DW_AT_external  (true)
                DW_AT_decl_file ("/path/to/tlsdbg.c")
                DW_AT_decl_line (1)

With an x86_64 target

0x00000023:   DW_TAG_variable
                DW_AT_name      ("foo")
                DW_AT_type      (0x00000036 "int")
                DW_AT_external  (true)
                DW_AT_decl_file ("/path/to/tlsdbg.c")
                DW_AT_decl_line (1)
                DW_AT_location  (DW_OP_const8u 0x0, DW_OP_GNU_push_tls_address)

The x86_64 relocs are

Relocation section '.rela.debug_info' at offset 0x400 contains 5 entries:
    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
0000000000000008  000000030000000a R_X86_64_32            0000000000000000 .debug_abbrev + 0
0000000000000011  000000040000000a R_X86_64_32            0000000000000000 .debug_str_offsets + 8
0000000000000015  000000070000000a R_X86_64_32            0000000000000000 .debug_line + 0
000000000000001f  000000060000000a R_X86_64_32            0000000000000000 .debug_addr + 8
000000000000002d  0000000a00000011 R_X86_64_DTPOFF64      0000000000000000 foo + 0

Note the R_X86_64_DTPOFF64