gdb: assign a valid section in convert_address_location_to_sals

The convert_address_location_to_sals function builds a symtab_and_line
from an explicit pc. Unless overlay debugging is enabled, the sal does not
contain a valid section (as find_pc_overlay will simply return nullptr).

While it is usually not a problem (as the sal users often recompute the
proper section, when needed), it may lead to the proper gdbarch not
being assigned when setting a breakpoint.

In code_breakpoint::add_location, gdb attempts to retrieve the gdbarch
through get_sal_arch by checking for the section or the symtab. However,
neither are currently set by cinvert_address_location_to_sals if the
debug symbols cannot be found. We then fall back to the current
architecture, which may cause errors in heterogeneous programs
(in ROCm, a breakpoint was not being hit since GDB was setting an
x86 int3 instruction instead of the architecture-appropriate s_trap 1).

This is a rework of a patch that was approved, but never merged
upstream (https://inbox.sourceware.org/gdb-patches/20241108195257.485488-2-lancelot.six@amd.com/).
The original change proposed to set the objfile field in the sal, and
check this field in get_sal_arch() if neither the section, nor the
symtab is defined. This patch makes GDB compute the section from the pc
instead of checking from the objfile in get_sal_arch, in accordance with
the rule of trying to set the section when creating the sal implemented
in this patch series. The test cases from the original patch are
included in this new one.

This should have minimal impact on other parts of GDB as users of this
section field would either (1) recompute it the same way (2) not use it
at all. In the case of overlay debugging, then the preceding call to
find_pc_overlay would likely assign a section.

Co-Authored-By: Lancelot SIX <lancelot.six@amd.com>
Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I23cef6ad5a66f696536c7c49c885a074bfea9b23