Support Linux binfmt_misc and APE loading on Apple

The "no modify self" variant of Actually Portable Executable is now
supported on all platforms. If you use `$(APE_NO_MODIFY_SELF)` then
ld.bfd will embed a 4096 byte ELF binary and a 4096 byte Macho file
which are installed on the fly to ${TMPDIR:-/tmp}, which enables us
launch the executable, without needing to copy the whole executable

To prevent it from copying a tiny executable to your temp directory
you need to install the `ape` command (renamed from ape-loader), to
a system path. For example:

    # FreeBSD / NetBSD / OpenBSD
    make -j8 o//ape/ape
    cp o//ape/ape /usr/bin/ape

    # Mac OS
    # make -j8 o//ape/ape.macho
    curl https://justine.lol/ape.macho >/usr/bin/ape
    chmod +x /usr/bin/ape

On Linux you can get even more performance with the new binfmt_misc
support which makes launching non-modifying APE binaries as fast as
launching ELF executables. Running the following command:

    # Linux
    ape/apeinstall.sh

Will copy APE loader to /usr/bin/ape and register with binfmt_misc
Lastly, this change also fixes a really interesting race condition
with OpenBSD thread joining.

ape/loader.lds

@@ -24,9 +24,18 @@ SECTIONS {
   .text : {
     *(.text)
     *(.rodata .rodata.*)
+    . = ALIGN(4096);
   }
   filesz = . - ehdr;
+  .bss ALIGN(4096) : {
+    bss = .;
+    *(.bss)
+    . = ALIGN(4096);
+  }
+  memsz = . - ehdr;
   /DISCARD/ : {
     *(.*)
   }
 }
+
+bsssize = SIZEOF(.bss);