[gdb/symtab] Handle DW_FORM_strx in form_requires_reprocessing

[gdb/symtab] Handle DW_FORM_strx in form_requires_reprocessing

When running test-case gdb.dwarf2/fission-mix.exp using gcc-11, I run into:
...
(gdb) file fission-mix^M
Reading symbols from fission-mix...^M
src/gdb/dwarf2/attribute.h:258: internal-error: \
  void attribute::set_unsigned_reprocess(ULONGEST): \
  Assertion `form_requires_reprocessing ()' failed.^M
...

This happens when calling set_unsigned_reprocess on an attribute with form
DW_FORM_strx.  The assert triggers because DW_FORM_strx is not listed in
form_requires_reprocessing.

Fix this by adding DW_FORM_strx in form_requires_reprocessing.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-02-11  Tom de Vries  <tdevries@suse.de>

	PR symtab/27353
	* dwarf2/attribute.c (attribute::form_requires_reprocessing):
	Return true for DW_FORM_strx.