gh-153285: Preserve LTO target names on macOS

[vismaytiwari]

Fixes #153285.

On macOS with --with-lto, the LTO linker flags include Make automatic target variable $@. PY_CORE_EXE_LDFLAGS used an immediate assignment, so that variable expanded while Make parsed the file rather than while an executable target was linked. This produced an empty object path for Programs/_testembed and caused test_embed failures.

Use a recursive assignment so $@ is evaluated in the link recipe.

Validation:

• Reproduced on macOS 26.5 with Apple Clang 17 using ./configure --with-lto and make -j8: test_embed had 84 failures before the change.
• Fresh patched ThinLTO build: test_embed passed 90 tests with 8 expected skips.
• Full ThinLTO suite: 50,091 tests run; 464 test files passed, with 32 platform skips and 9 resource skips.
• make patchcheck passed.

• Issue: Compiling --with-lto results in test_embed failures on Darwin #153285

[python-cla-bot]

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