www-client/firefox: block USE=pgo when USE=-clang

www-client/firefox: block USE=pgo when USE=-clang

USE=pgo is currently known to be broken when using GCC.

Instead of silently ignoring USE=pgo when using GCC, we are
forcing a user decision so user has the chance to enable USE=clang
in case PGO is important for user without wasting 2+ hours
commpiling firefox just to find out that PGO which was requested
was ignored.

Bug: https://bugs.gentoo.org/709594
Bug: https://bugs.gentoo.org/709654
Package-Manager: Portage-2.3.89, Repoman-2.3.20
Signed-off-by: Thomas Deutschmann <whissi@gentoo.org>