Unbreak zstd on strict alignment architectures

zstd is broken on sparc64 due to unaligned accesses resulting in bus
errors. Since __GNUC__ is defined and MEM_FORCE_MEMORY_ACCESS isn't
defined, it defaults to 1, and we run into these:

https://github.com/facebook/zstd/blob/1be95291a89160be121c987c2e385331a65a4a0e/lib/common/mem.h#L192-L199

One simple fix is to define MEM_FORCE_MEMORY_ACCESS to 0 via CPPFLAGS.
This forces all platforms to use memcpy, which may or may not result in
some slowdown. We can consider more elaborate platform-specific fixes if
that is desired, but then I need instructions.

With this diff, the cmake test for zstd doesn't abort with a bus error,
which allows me to build qt6/qtbase on sparc64, and in turn we should
get a sizable chunk of the tree back to building.

Index: Makefile
===================================================================
RCS file: /cvs/ports/archivers/zstd/Makefile,v
retrieving revision 1.43
diff -u -p -r1.43 Makefile
--- Makefile 11 Feb 2023 22:04:54 -0000 1.43
+++ Makefile 3 Mar 2023 00:06:05 -0000
@@ -2,6 +2,7 @@ COMMENT = zstandard fast real-time comp

V = 1.5.4
DISTNAME = zstd-${V}
+REVISION = 0

SHARED_LIBS = zstd 6.2 # 1.5.4

@@ -23,12 +24,15 @@ LIB_DEPENDS = archivers/lz4 \
BUILD_DEPENDS = sysutils/ggrep

MAKE_ENV = CC="${CC}" \
- CPPFLAGS="-I${LOCALBASE}/include" \
+ CPPFLAGS="${CPPFLAGS} -I${LOCALBASE}/include" \
LDFLAGS="${LDFLAGS} -L${LOCALBASE}/lib"
MAKE_FLAGS = SHARED_EXT_VER="so.$(LIBzstd_VERSION)" \
SONAME_FLAGS= \
V=1
FAKE_FLAGS = PREFIX="${PREFIX}"
+
+# Avoid unaligned access; use memcpy.
+CPPFLAGS += -DMEM_FORCE_MEMORY_ACCESS=0

USE_GMAKE = Yes