rust: disable executable-no-mangle-strip test on PowerPC due to linker issue

rust: disable executable-no-mangle-strip test on PowerPC due to linker issue

The executable-no-mangle-strip test fails on PowerPC target with an undefined
reference error when linking. The linker attempts to resolve this undefined symbol
at link time, causing the test to fail with:
  undefined reference to `THIS_SYMBOL_SHOULD_BE_UNDEFINED'

The test is designed to verify garbage collection behavior of #[no_mangle]
functions and used(compiler)] statics. This appears to be a PowerPC-specific
linker behavior issue.

Temporarily disable this test on PowerPC until the underlying issue is
resolved upstream.

Related upstream issue: https://github.com/rust-lang/rust/issues/144777

Fixes oe-selftest failure:
- tests/ui/linking/executable-no-mangle-strip.rs on powerpc-poky-linux-gnu

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>