Fix register regression in DWARF evaluator

Fix register regression in DWARF evaluator

On an internal test case, using an arm-elf target, commit ba5bc3e5a92
("Make DWARF evaluator return a single struct value") causes a
regression.  (It doesn't happen for any of the other cross targets
that I test when importing upstream gdb.)

I don't know if there's an upstream gdb test case showing the same
problem... I can only really run native tests with dejagnu AFAIK.

The failure manifests like this:

    Breakpoint 1, file_1.export_1 (param_1=<error reading variable: Unable to access DWARF register number 64>, str=...) at [...]/file_1.adb:5

Whereas when it works it looks like:

    Breakpoint 1, file_1.export_1 (param_1=99.0, str=...) at [...]/file_1.adb:5

The difference is that the new code uses the passed-in gdbarch,
whereas the old code used the frame's gdbarch, when handling
DWARF_VALUE_REGISTER.

This patch restores the use of the frame's arch.