[gdb/testsuite] Compile dwzbuildid-mismatch more quietly

[gdb/testsuite] Compile dwzbuildid-mismatch more quietly

When running test-case gdb.dwarf2/dwzbuildid.exp with target board
cc-with-gdb-index, we have:
...
Running src/gdb/testsuite/gdb.dwarf2/dwzbuildid.exp ...
gdb compile failed, warning: File "dwzbuildid5.o" has a different \
  build-id, file skipped
could not find '.gnu_debugaltlink' file for dwzbuildid-mismatch
warning: File "dwzbuildid5.o" has a different build-id, file skipped
Error while writing index for `dwzbuildid-mismatch': could not find \
  '.gnu_debugaltlink' file for dwzbuildid-mismatch
...
and likewise for target board cc-with-debug-names.

These are gdb-add-index warnings and errors due to the executable
dwzbuildid-mismatch having a build-id mismatch.

Be less verbose by adding "quiet" to the compile flags.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-04-24  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/dwzbuildid.exp: Add quiet to dwzbuildid-mismatch compile
	flags.