bsd.sys.mk: Silence a few GCC warnings for C++

These warnings were already marked with -Wno-error=, but they trigger
so many false positives in libc++ headers to be utterly useless for
C++ code, so disable them entirely for C++.

Reviewed by:	imp, dim
Differential Revision:	https://reviews.freebsd.org/D54867

(cherry picked from commit 6fb87a1a6042887550571316fa9e539239f158d1)

share/mk/bsd.sys.mk

@@ -190,6 +190,11 @@ CWARNFLAGS+=	-Wno-error=address			\
 		-Wno-error=uninitialized		\
 		-Wno-error=unused-function		\
 		-Wno-error=unused-value
+
+# These warnings are raised by headers in libc++ so are disabled
+# globally for all C++
+CXXWARNFLAGS+=	-Wno-attributes				\
+		-Wno-deprecated-declarations
 .endif
 
 # GCC 6.1.0
@@ -243,6 +248,7 @@ CWARNFLAGS+=	-Wno-error=overflow
 # globally for all C++
 CXXWARNFLAGS+=	-Wno-literal-suffix			\
 		-Wno-c++20-extensions			\
+		-Wno-attributes				\
 		-Wno-error=unknown-pragmas
 .endif