[NVPTX] Fix LIT tests with default nameTableKind

[NVPTX] Fix LIT tests with default nameTableKind

Default nameTableKind results in the following DWARF section:

    .section .debug_pubnames
    {
      .b32 LpubNames_end0-LpubNames_start0    // Length of Public Names Info
      LpubNames_start0:
      [...]
      LpubNames_end0:
    }

Without -mattr=+ptx75 ptxas complains about labels and label
expressions:

error   : Feature 'labels1 - labels2 expression in .section' requires
PTX ISA .version 7.5 or later
error   : Feature 'Defining labels in .section' requires PTX ISA
.version 7.0 or later

The patch modifies dbg-value-const-byref.ll to let it run without PTX
7.5 (available from CUDA 11.0), and adds a new test just for this
case.

Differential revision: https://reviews.llvm.org/D124108