testsuite: fix dwarf2/utf-1.C with DWARF4

testsuite: fix dwarf2/utf-1.C with DWARF4

Running
$ make check-c++ RUNTESTFLAGS='--target_board=unix\{-gdwarf-5,-gdwarf-4\} dwarf2.exp=utf-1.C'
shows
FAIL: g++.dg/debug/dwarf2/utf-1.C  -std=gnu++20  scan-assembler-times DW_AT_encoding \\(0x10\\) 3
because with -gdwarf-4 the output is:

  .byte   0x10    # DW_AT_encoding

but with -gdwarf-5 the output is the expected:

                # DW_AT_encoding (0x10)

The difference is caused by the DWARF5 optimize_implicit_const
optimization:
<https://gcc.gnu.org/pipermail/gcc-patches/2016-October/459762.html>

I suppose we could do what testsuite/rust/debug/chartype.rs does
and just run the test with -gdwarf-4.

gcc/testsuite/ChangeLog:

	* g++.dg/debug/dwarf2/utf-1.C: Use -gdwarf-4.  Adjust expected
	output.