cmd/link: select DWARF 4 when using external linker

[thanm]

When a user requests external linkage, the Go linker seems to be invoking the external linker with the "-gdwarf-2" flag (asking for DWARF version 2). Example:

$ go build -x -ldflags "-v -linkmode external -extld gcc" himom.go 1> transcript.txt 2>&1
$ fgrep 'host link' transcript.txt  
0.07 host link: "gcc" "-m64" "-gdwarf-2" "-o" "/tmp/go-build530006148/b001/exe/a.out" ...
$ 

Given that the DWARF produced by the Go compiler is now DWARF 4, I don't think it makes sense to continue to use this flag. When I look at the dwarf from the himom.go build above, it appears that all compilation units in the .debug_info are showing version 4 except for this one:

  Compilation Unit @ offset 0x5b7c5:
   Length:        0x4f (32-bit)
   Version:       2
   Abbrev Offset: 0x1141
   Pointer Size:  8
 <0><5b7d0>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <5b7d1>   DW_AT_stmt_list   : 0x110fa	
    <5b7d5>   DW_AT_low_pc      : 0x488867	
    <5b7dd>   DW_AT_high_pc     : 0x48887e	
    <5b7e5>   DW_AT_name        : gcc_amd64.S	
    <5b7f1>   DW_AT_comp_dir    : /x/go/src/runtime/cgo	
    <5b80a>   DW_AT_producer    : GNU AS 2.24	
    <5b816>   DW_AT_language    : 32769	(MIPS assembler)

Not sure what the original thinking was for doing this, but it doesn't seem useful now.

[gopherbot]

Change https://golang.org/cl/73672 mentions this issue: cmd/link: don't pass -gdwarf-2 to external linker

[ianlancetaylor]

For the record, we started passing -gdwarf-2 in https://golang.org/cl/7891044. But I don't really understand why we are passing a -g option here at all, as we are only passing object files, and the linker just ignores the -g option anyhow. So I think your CL is right.