flag-o-matic.eclass: add LDFLAGS testing against linker

flag-o-matic.eclass: add LDFLAGS testing against linker

Before the change we tested only compiler driver (gcc flag parser)
for LDFLAGS.

This does not cover cases when we would really like to filter out
unsupported linker flags like -Wl,--hash-style=gnu passed to non-ELF
targets.

The change adds test-flag-CCLD() helper to perform all of assembly,
compilation and linking steps. Helper is used to filter LDFLAGS variable
in strip-unsupported-flags().

Closes: https://bugs.gentoo.org/333763
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>