gccrs: Emit block scope drops through TRY_FINALLY_EXPR

[Lishin1215]

This PR consists of 3 commits.

It changes block-scope Drop emission to use a TRY_FINALLY_EXPR cleanup instead of
appending drop calls directly to the block body.

For example, a block like:

{
    let x = Droppable;
}

is lowered as:

try {
    let x = Droppable;
} finally {
    drop(x);
}

This follows the same general cleanup/finally shape used by the Go frontend for defer handling.

For now, this PR only covers block-scope drops. Function-scope drops, explicit return cleanup,
break/continue are left for follow-up work.

---

commit 1

gccrs: Add helper to build scope drop cleanup

gcc/rust/ChangeLog:

* backend/rust-compile-drop.cc
(CompileDrop::build_current_scope_drop_cleanup): New function to
build current scope drop calls as a statement tree.
(CompileDrop::emit_current_scope_drop_calls): Use it.
* backend/rust-compile-drop.h
(CompileDrop::build_current_scope_drop_cleanup): Declare.

commit 2

gccrs: Emit block scope drops through TRY_FINALLY_EXPR

gcc/rust/ChangeLog:

* backend/rust-compile-block.cc
(CompileBlock::visit): Build current scope drop cleanup and pass it
when popping the block.
* backend/rust-compile-context.h
(Context::pop_block): Use pop_block_impl.
(Context::pop_block_with_cleanup): New function for popping a block
with cleanup code.
(Context::pop_block_impl): New helper that adds the block statements
directly, or wraps them in TRY_FINALLY_EXPR when cleanup is present.

commit 3

gccrs: Use EH_ELSE_EXPR in block Drop TRY_FINALLY_EXPR

gcc/rust/ChangeLog:

* backend/rust-compile-context.h
(Context::pop_block_impl): Wrap Drop cleanup in an EH_ELSE_EXPR.

---

Note:

TRY_FINALLY_EXPR cleanup originally caused drop-nested-block-scope.rs to fail
to link with an undefined reference to __gccrs_personality_v0.

This version wraps the cleanup operand in EH_ELSE_EXPR, using the Drop cleanup
as the normal cleanup and an empty exceptional cleanup for now.
With that, drop-nested-block-scope.rs links and passes locally.

[Lishin1215]

I followed the cleanup shape described in gccint 11.7.6, the TRY_FINALLY_EXPR
cleanup operand can be an EH_ELSE_EXPR selector.

I did not add the empty volatile asm used by rustc_codegen_gcc to keep the
EH_ELSE_EXPR around, because the normal cleanup side here contains the actual
Drop cleanup, and the relevant tests pass with an empty exceptional cleanup.