/proc/self/exe now is optional

... for the benefit of 'unshare', 'chroot', etc.
If not present, then upx run-time stub unmaps all pages of original main program.
https://github.com/upx/upx/issues/721

	modified:   stub/src/amd64-linux.elf-entry.S
	modified:   stub/src/amd64-linux.elf-fold.S
	modified:   stub/src/arm.v4a-linux.elf-entry.S
	modified:   stub/src/arm.v4a-linux.elf-fold.S
	modified:   stub/src/arm64-linux.elf-entry.S
	modified:   stub/src/arm64-linux.elf-fold.S
	modified:   stub/src/i386-linux.elf-fold.S
	modified:   stub/src/mipsel.r3000-linux.elf-fold.S
	modified:   stub/src/powerpc-linux.elf-fold.S
	modified:   stub/src/powerpc64le-linux.elf-fold.S
	modified:   ../misc/testsuite/upx_testsuite_1-expected_sha256sums.sh

           ... plus all associaed *.h, *.map, *.dump in stub/

src/stub/tmp/mips.r3000-linux.elf-fold.map

@@ -12,7 +12,7 @@ TARGET(elf32-bigmips)
 .text           0x0000000000100080      0xad0
  *(.text)
  .text          0x0000000000100080      0x1c0 tmp/mips.r3000-linux.elf-fold.o
-                0x0000000000100218                Pprotect
+                0x000000000010021c                Pprotect
  .text          0x0000000000100240      0x910 tmp/mips.r3000-linux.elf-main.o
                 0x00000000001009b0                upx_main
  *(.data)