[runtimes] Simplify testing configurations on GCC by using -nostdlib++ (#67021)

Since GCC now supports -nostdlib++, we can remove some complexity in the
test configurations and do the same as Clang. However, we can't fully
remove the GCC test configuration for libc++ because we apparently need
to explicitly link against libm for some tests to work.

GitOrigin-RevId: d217aeb0b7baad3bf92156b2e5ea1732ee8446a1
2 files changed