Don't use -arch flags when building gen_manual. It will be run on the
build machine so it needs to be built for the build machine's arch.
--- contrib/gen_manual/Makefile.orig	2022-08-15 17:45:31.000000000 -0500
+++ contrib/gen_manual/Makefile	2022-08-18 23:40:55.000000000 -0500
@@ -34,6 +34,7 @@
 CXXFLAGS += -Wall -Wextra -Wcast-qual -Wcast-align -Wshadow -Wstrict-aliasing=1 -Wswitch-enum -Wno-comment
 CPPFLAGS += $(MOREFLAGS)
 FLAGS   = $(CXXFLAGS) $(CPPFLAGS) $(LDFLAGS)
+FLAGS := $(filter-out -arch^%,$(subst -arch ,-arch^,$(FLAGS)))
 
 LZ4API = ../../lib/lz4.h
 LZ4MANUAL = ../../doc/lz4_manual.html