[gdb/backtrace] Fix printing of fortran string args

[gdb/backtrace] Fix printing of fortran string args

When running test-case gdb.fortran/mixed-lang-stack.exp, it passes, but we
find in gdb.log:
...
 (gdb) bt^M
   ...
 #7  0x000000000040113c in mixed_func_1b (a=1, b=2, c=3, d=(4,5), \
   e=<error reading variable: value requires 140737488341744 bytes, which \
   is more than max-value-size>, g=..., _e=6) at mixed-lang-stack.f90:87^M
...
while a bit later in gdb.log, we have instead for the same frame (after
adding a gdb_test_no_output "set print frame-arguments all" to prevent
getting "e=..."):
...
 (gdb) up^M
 #7  0x000000000040113c in mixed_func_1b (a=1, b=2, c=3, d=(4,5), \
   e='abcdef', g=( a = 1.5, b = 2.5 ), _e=6) at mixed-lang-stack.f90:87^M
...

The difference is that in the latter case, we print the frame while it's
selected, while in the former, it's not.

The problem is that while trying to resolve the dynamic type of e in
resolve_dynamic_type, we call dwarf2_evaluate_property with a frame == NULL
argument, and then use the selected frame as the context in which to evaluate
the dwarf property, effectively evaluating a DW_OP_fbreg operation in the
wrong frame context.

Fix this by temporarily selecting the frame of which we're trying to print the
arguments in print_frame_args, borrowing code from print_frame_local_vars that
was added to fix a similar issue in commit 16c3b12f19 "error/internal-error
printing local variable during "bt full".

Build and tested on x86_64-linux.

gdb/ChangeLog:

2020-08-15  Tom de Vries  <tdevries@suse.de>

	PR backtrace/26390
	* stack.c (print_frame_args): Temporarily set the selected
	frame to FRAME while printing the frame's arguments.

gdb/testsuite/ChangeLog:

2020-08-15  Tom de Vries  <tdevries@suse.de>

	PR backtrace/26390
	* gdb.fortran/mixed-lang-stack.exp: Call bt with -frame-arguments all.
	Update expected pattern.