build: always specify PORTABLE=1 when building RocksDB

Something recently changed on the TC agents (presumably usage of a new
machine type with a different CPU) so that using `-march=native` when
building RocksDB causes AVX512 instructions to be used which are not
present on roachprod machines. The result is that binaries built in this
way will die with "illegal instruction". RocksDB has a mechanism to
disable the use of `-march=native`: pass `PORTABLE=1` to the make
command. We were already doing this for release builds. Now we're doing
it for all builds.

Fixes cockroachdb#58754

Release note: none

Makefile

@@ -638,7 +638,7 @@ $(ROCKSDB_DIR)/Makefile: $(C_DEPS_DIR)/rocksdb-rebuild | bin/.submodules-initial
 	@# NOTE: If you change the CMake flags below, bump the version in
 	@# $(C_DEPS_DIR)/rocksdb-rebuild. See above for rationale.
 	cd $(ROCKSDB_DIR) && CFLAGS+=" $(sse)" && CXXFLAGS+=" $(sse)" && cmake $(xcmake-flags) $(ROCKSDB_SRC_DIR) \
-	  $(if $(findstring release,$(BUILDTYPE)),-DPORTABLE=ON) -DWITH_GFLAGS=OFF \
+	  -DPORTABLE=ON -DWITH_GFLAGS=OFF \
 	  -DSNAPPY_LIBRARIES=$(LIBSNAPPY) -DSNAPPY_INCLUDE_DIR="$(SNAPPY_SRC_DIR);$(SNAPPY_DIR)" -DWITH_SNAPPY=ON \
 	  $(if $(use-stdmalloc),,-DJEMALLOC_LIBRARIES=$(LIBJEMALLOC) -DJEMALLOC_INCLUDE_DIR=$(JEMALLOC_DIR)/include -DWITH_JEMALLOC=ON) \
 	  -DCMAKE_BUILD_TYPE=$(if $(ENABLE_ROCKSDB_ASSERTIONS),Debug,Release) \