mlton-script: OpenBSD link-opt for no xonly text

Add a `-target-link-opt` for OpenBSD that disables "xonly" text.

OpenBSD's linker, by default, makes a program's text segment
executable-only ("xonly"): that is, executable but not readable.
However, MLton needs to be able to read the text segment when
mapping return addresses to stack frame indices, possibly among
other things.  This causes programs compiled with MLton's native
code generator to fault and die with `SIGSEGV`.

However, the default "xonly" behavior can be disabled with a
linker option (`--no-execute-only`), so we can turn it off with
a `-target-link-opt` specific to OpenBSD in `mlton-script`.
Binaries built in this configuration run as expected.

Fixes #538

Signed-off-by: Dan Cross <cross@gajendra.net>

bin/mlton-script

@@ -121,6 +121,7 @@ for arg in "$@"; do
                -target-link-opt mingw                                   \
                        '-lws2_32 -lkernel32 -lpsapi -lnetapi32 -lwinmm' \
                -target-link-opt mingw '-Wl,--enable-stdcall-fixup'      \
+               -target-link-opt openbsd '-Wl,--no-execute-only'         \
                -target-link-opt solaris '-lnsl -lsocket -lrt'           \
                -target-link-opt x86 '-m32'                              \
                -profile-exclude '\$\(SML_LIB\)'