Fix GCC warnings causing build failure after r293724

Disable some compiler warnings for GCC (non-standard compiler) fixing
build failures introduced by r293724, which enabled WARNS in the EFI boot
code, when compiling with none standard compiler (GCC).

Raised by:	ian
MFC after:	2 weeks
X-MFC-With:	r293268
Sponsored by:	Multiplay

sys/boot/arm/uboot/Makefile

@@ -8,6 +8,8 @@ NEWVERSWHAT=	"U-Boot loader" ${MACHINE_ARCH}
 BINDIR?=	/boot
 INSTALLFLAGS=	-b
 WARNS?=		1
+CWARNFLAGS.gcc+=	-Wno-int-to-pointer-cast
+
 # Address at which ubldr will be loaded.
 # This varies for different boards and SOCs.
 UBLDR_LOADADDR?=	0x1000000

sys/boot/efi/fdt/Makefile

@@ -7,6 +7,8 @@
 LIB=		efi_fdt
 INTERNALLIB=
 WARNS?=		6
+CWARNFLAGS.gcc+=	-Wno-strict-prototypes
+CWARNFLAGS.gcc+=	-Wno-redundant-decls
 
 SRCS=		efi_fdt.c
 

sys/boot/efi/libefi/Makefile

@@ -3,6 +3,7 @@
 LIB=	efi
 INTERNALLIB=
 WARNS?=	2
+CWARNFLAGS.gcc+= -Wno-attributes
 
 SRCS=	delay.c efi_console.c efinet.c efipart.c errno.c handles.c \
 	libefi.c time.c