config: don't force shared linkage on FreeBSD

-shared was hardcoded, so when building with --disable-shared it amounts
to trying to do shared linkage against static libs, which naturally
fails.

The fix is straightforward; just don't hardcode it. libtool will work
out what to do.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16427

config/Rules.am

@@ -57,7 +57,7 @@ AM_LDFLAGS += $(ASAN_LDFLAGS)
 AM_LDFLAGS += $(UBSAN_LDFLAGS)
 
 if BUILD_FREEBSD
-AM_LDFLAGS += -fstack-protector-strong -shared
+AM_LDFLAGS += -fstack-protector-strong
 AM_LDFLAGS += -Wl,-x -Wl,--fatal-warnings -Wl,--warn-shared-textrel
 AM_LDFLAGS += -lm
 endif