Imported from GitHub PR https://github.com/abseil/abseil-cpp/pull/1738

Since #1707 and #1710, `-Xarch_` option groups are present as single elements in the libraries' COPTS, and a `SHELL:` prefix is added to each of them. While these addressed certain build issues, they broke the handling of `-Xarch_` option groups in pkgconfig files.

Fix that by taking care of the `SHELL:` prefix in COPTS when generating pkgconfig files. The skip-next-flag mechanism is also removed as the option groups are now present as single elements.

Merge fd1246acbf4052d66fef66ee195cf254bca35b6c into 08850701

Merging this change closes #1738

COPYBARA_INTEGRATE_REVIEW=https://github.com/abseil/abseil-cpp/pull/1738 from ZhongRuoyu:pkgconfig-xarch-handling fd1246acbf4052d66fef66ee195cf254bca35b6c
PiperOrigin-RevId: 660055129
Change-Id: I57a1a51b2eb8983d076a3d5336acef12acae4560