gerbil @2020-05-06-5a7705f: build failure on arm64

[ryandesign (Ryan Carsten Schmidt)]

gerbil fails to build on arm64:

​https://build.macports.org/builders/ports-13_arm64-builder/builds/36181/steps/install-port/logs/stdio

clang: warning: argument unused during compilation: '-msse2' [-Wunused-command-line-argument]

/Library/Developer/CommandLineTools/usr/lib/clang/14.0.3/include/mmintrin.h:14:2: error: "This header is only meant to be used on x86 and x64 architecture"
#error "This header is only meant to be used on x86 and x64 architecture"
 ^