nptl: Add more missing placeholder abi symbol from nanosleep move

nptl: Add more missing placeholder abi symbol from nanosleep move

This patch adds the missing __libpthread_version_placeholder for
GLIBC_2.2.6 version from the nanosleep implementation move from
libpthread to libc (79a547b162).

It also fixes the wrong compat symbol definitions added by changing
back the version used on vfork check and remove the
__libpthread_version_placeholder added on some ABI (4f4bb489e0dd).

The __libpthread_version_placeholder is also refactored to make it
simpler to add new compat_symbols by adding a new macro
compat_symbol_unique which uses the compiler extension __COUNTER__
to generate unique strong alias to be used with compat_symbol.

Checked with a updated-abi on the all affected abis of the nanosleep
move.

Change-Id: I347a4dbdc931bb42b359456932dd1e17aa4d4078