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 2d51acd2fb

(From OE-Core rev: 2435b16b050ebdca0f70fe9ed1eebfdd3bacf19d)

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>

meta/recipes-devtools/rust/rust-source.inc

@@ -11,6 +11,7 @@ SRC_URI += "https://static.rust-lang.org/dist/rustc-${RUST_VERSION}-src.tar.xz;n
             file://0001-Disable-libunwind-cross-architecture-unwinding.patch;patchdir=${RUSTSRC} \
             file://triagebot.patch;patchdir=${RUSTSRC} \
             file://0001-riscv32-Define-plain-syscalls-as-their-time64-varian.patch;patchdir=${RUSTSRC} \
+            file://backport-fix-test-string-merging.patch;patchdir=${RUSTSRC} \
 "
 SRC_URI[rust.sha256sum] = "0c1dcbb4f762513d021e1a282c0ac58c0a423642b3a6bf581cafb5414df4193e"