Disable -Wreturn-type on GCC.

GCC is more pedantic than clang about warning when a function doesn't
handle undefined enum values (see GCC bug 87950). Clang's warning
gives a more pragmatic coverage and should find any real bugs, so
disable the warning for GCC rather than adding __unreachable
annotations to appease GCC.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D34147