devel/mdb: fix tautological-pointer-compare warning option

During an exp-run for llvm 12 (see bug 255570), it turned out that
devel/mdb does not build with clang 12.0.0:

cc  -O2 -pipe  -DMDB_PREFIX=\"/usr/local\" -g -fstack-protector-strong -fno-strict-aliasing  -I/wrkdirs/usr/ports/devel/mdb/work/illumos-gate-60ff9f11c2/freebsd/cddl/usr.bin/mdb/mdb/../../../../sys/cddl/compat/opensolaris  -I/usr/src/sys/cddl/compat/opensolaris  -I/wrkdirs/usr/ports/devel/mdb/work/illumos-gate-60ff9f11c2/freebsd/cddl/usr.bin/mdb/mdb/../../../../cddl/compat/opensolaris/include  -I/usr/src/cddl/compat/opensolaris/include  -I/usr/src/cddl/contrib/opensolaris/head  -I/wrkdirs/usr/ports/devel/mdb/work/illumos-gate-60ff9f11c2/freebsd/cddl/usr.bin/mdb/mdb/../../../../../usr/src/lib/libdisasm/common  -I/wrkdirs/usr/ports/devel/mdb/work/illumos-gate-60ff9f11c2/freebsd/cddl/usr.bin/mdb/mdb/../../../../../usr/src/common/dis/i386  -I/usr/src/cddl/contrib/opensolaris/lib/libctf/common  -I/usr/src/sys/cddl/contrib/opensolaris/uts/common -I/wrkdirs/usr/ports/devel/mdb/work/illumos-gate-60ff9f11c2/freebsd/cddl/usr.bin/mdb/mdb/../../../../../usr/src/cmd/mdb/intel/mdb -I/wrkdirs/usr/ports/devel/mdb/work/illumos-gate-60ff9f11c2/freebsd/cddl/usr.bin/mdb/mdb/../../../../../usr/src/cmd/mdb/intel/amd64/mdb -I/wrkdirs/usr/ports/devel/mdb/work/illumos-gate-60ff9f11c2/freebsd/cddl/usr.bin/mdb/mdb/../../../../../usr/src/cmd/mdb/intel -I/wrkdirs/usr/ports/devel/mdb/work/illumos-gate-60ff9f11c2/freebsd/cddl/usr.bin/mdb/mdb/../../../../../usr/src/lib/libsaveargs/common -D_MDB -I/wrkdirs/usr/ports/devel/mdb/work/illumos-gate-60ff9f11c2/freebsd/cddl/usr.bin/mdb/mdb/../../../../../usr/src/cmd/mdb/common -I/usr/src -DDIS_TEXT -D_SYS_INT_TYPES_H -Dmemalign=aligned_alloc -DTCGETS=TIOCGETA -DTCSETSW=TIOCSETAW -DIUCLC=0 -DENODATA=ENOENT   -D__amd64 -DIN_BASE -DNEED_SOLARIS_BOOLEAN -fPIE -MD  -MF.depend.kvm_amd64dep_freebsd.o -MTkvm_amd64dep_freebsd.o -std=gnu99 -Wno-format-zero-length -nobuiltininc -idirafter /usr/lib/clang/12.0.0/include -fstack-protector-strong -Wsystem-headers -Werror -Wall -Wno-format-y2k -W -Wno-unused-parameter -Wstrict-prototypes -Wmissing-prototypes -Wpointer-arith -Wreturn-type -Wcast-qual -Wwrite-strings -Wswitch -Wshadow -Wunused-parameter -Wcast-align -Wchar-subscripts -Wnested-externs -Wredundant-decls -Wold-style-definition -Wno-pointer-sign -Wno-unknown-pragmas -Wmissing-variable-declarations -Wthread-safety -Wno-empty-body -Wno-string-plus-int -Wno-unused-const-variable -Wno-bitwise-op-parentheses  -Wno-cast-align  -Wno-cast-qual  -Wno-char-subscripts  -Wno-incompatible-pointer-types  -Wno-incompatible-pointer-types-discards-qualifiers  -Wno-logical-op-parentheses  -Wno-missing-braces  -Wno-missing-field-initializers  -Wno-missing-variable-declarations  -Wno-shadow  -Wno-sign-compare  -Wno-strict-prototypes  -Wno-error-tautological-pointer-compare  -Wno-unused-parameter  -Qunused-arguments    -c /wrkdirs/usr/ports/devel/mdb/work/illumos-gate-60ff9f11c2/freebsd/cddl/usr.bin/mdb/mdb/../../../../../usr/src/cmd/mdb/intel/mdb/kvm_amd64dep_freebsd.c -o kvm_amd64dep_freebsd.o
error: unknown -Werror warning specifier: '-Wno-error-tautological-pointer-compare' [-Werror,-Wunknown-warning-option]
*** [kvm_amd64dep_freebsd.o] Error code 1

This is because for a while -Wno-error- options should be spelled as
-Wno-error=, instead.

Approved by:	maintainer timeout (2 weeks)
PR:		255955
MFH:		2021Q2

devel/mdb/Makefile

@@ -1,5 +1,6 @@
 PORTNAME=	mdb
 PORTVERSION=	20210125
+PORTREVISION=	1
 CATEGORIES=	devel
 
 MAINTAINER=	jhb@FreeBSD.org

devel/mdb/files/patch-mdb_Makefile

@@ -0,0 +1,11 @@
+--- mdb/Makefile.orig	2021-01-25 20:32:57 UTC
++++ mdb/Makefile
+@@ -201,7 +201,7 @@ CWARNFLAGS.clang+= \
+ 	-Wno-shadow \
+ 	-Wno-sign-compare \
+ 	-Wno-strict-prototypes \
+-	-Wno-error-tautological-pointer-compare \
++	-Wno-error=tautological-pointer-compare \
+ 	-Wno-unused-parameter
+
+ .if ${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} >= 100000 || \