Check for OS instead of the architecture in Makefile

The linking issue that prompted the addition of the conditional check
was due to the different compiler toolchain used on Linux (gcc) vs macos (clang).

Linux can run on arm architecture too, but passing the `-undefined dynamic_lookup` flag
to `gcc` would cause it to fail:

```
$ make
cc -O3 -Wall -I/home/alco/.asdf/installs/erlang/25.3.2.2/erts-13.2.2.1/include \
   -Ic_src/libpg_query -fPIC -lpthread -shared -undefined dynamic_lookup \
   -o priv/libpg_query_ex.so c_src/libpg_query_ex.c c_src/libpg_query/libpg_query.a
/usr/bin/ld: cannot find dynamic_lookup: No such file or directory
collect2: error: ld returned 1 exit status
make: *** [Makefile:21: priv/libpg_query_ex.so] Error 1
```

Author: alco

Makefile

@@ -5,9 +5,9 @@ CFLAGS += -I$(ERLANG_PATH)
 LIBPG_QUERY_PATH = c_src/libpg_query
 
 CFLAGS += -I$(LIBPG_QUERY_PATH) -fPIC
-UNAME_P := $(shell uname -p)
+
 LDFLAGS = -lpthread
-ifneq ($(filter arm%,$(UNAME_P)),)
+ifeq ($(shell uname -s),Darwin)
     LDFLAGS += -undefined dynamic_lookup
 endif