[PATCH] um: fix CONFIG_GCOV for built-in code

[Mariia Nikitash]

Hi Alex,

I tested this while investigating GCC/GCOV coverage under UML using KUnit.

Tested on:
  Architecture: x86_64 UML
  Kernel: v7.3-rc3-61-gbc77531b2a6f
  KUnit configs: default.config + coverage_uml.config
  Compilers: GCC 13.3.0 and GCC 16.2.0

With GCC 13.3, the unpatched tree produced 731 .gcno files and
0 .gcda files after the KUnit run. With the patch applied, the
same run produced 731 .gcno files and 704 .gcda files, and LCOV
was able to generate coverage successfully.

I also reproduced the same result with GCC 16.2:

  without patch: 731 .gcno / 0 .gcda
  with patch:    731 .gcno / 704 .gcda

LCOV coverage generation also completed successfully with GCC 16.2.

Tested-by: Mariia Nikitash <nikitash.mariiaw at gmail.com>

Thanks,
Mariia