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

share/mk/bsd.sys.mk

@@ -206,6 +206,10 @@ CWARNFLAGS+=	-Wno-error=overflow
 .endif
 .endif
 
+# GCC produces false positives for functions that switch on an
+# enum (GCC bug 87950)
+CWARNFLAGS+=	-Wno-return-type
+
 # GCC's own arm_neon.h triggers various warnings
 .if ${MACHINE_CPUARCH} == "aarch64"
 CWARNFLAGS+=	-Wno-system-headers

sys/conf/kern.mk

@@ -75,6 +75,10 @@ CWARNEXTRA+=	-Wno-error=packed-not-aligned
 CWARNEXTRA+=	-Wno-address-of-packed-member			\
 		-Wno-error=alloca-larger-than=
 .endif
+
+# GCC produces false positives for functions that switch on an
+# enum (GCC bug 87950)
+CWARNFLAGS+=	-Wno-return-type
 .endif	# gcc
 
 # This warning is utter nonsense