rust: backport cstring-merging test fix for PowerPC targets

rust: backport cstring-merging test fix for PowerPC targets

The Rust cstring-merging assembly test was failing on PowerPC targets due to
LLVM's GlobalMerge optimization pass merging all global string constants into
a single merged global. This caused the test's CHECK directives to fail as
they expected separate .Lanon labels for each string literal ("foo", "bar",
"baz").

The test verifies that C-style string literals are correctly placed in
mergeable read-only sections (.rodata.str1.1) with proper section flags
(aMS - Allocatable, Merge, Strings) for linker optimization.

This backports the upstream fix that disables the GlobalMerge pass
(-Cllvm-args=-enable-global-merge=0) specifically for this test, allowing
it to verify the expected assembly structure without interference from
the optimization.

Fixes test failure:
- tests/assembly/cstring-merging.rs on powerpc-poky-linux-gnu

Backport of https://github.com/rust-lang/rust/commit/2d51acd2fbcbadb6f30709c5dd305494d413d388

Signed-off-by: Peter Tatrai <peter.tatrai.ext@siemens.com>
Tested-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>