/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/arm64-linux.elf-entry.bin.dump

@@ -13,7 +13,7 @@ Idx Name          Size      VMA               LMA               File off  Algn
   8 LZMA_DEC10    04a0  0  0  0eec  2**0  CONTENTS
   9 LZMA_DEC30    0  0  0  0138c  2**0  CONTENTS
  10 ELFMAINY      03e  0  0  0138c  2**0  CONTENTS
- 11 ELFMAINZ      0138  0  0  013cc  2**2  CONTENTS
+ 11 ELFMAINZ      0110  0  0  013cc  2**2  CONTENTS
 SYMBOL TABLE:
 0000000000000000 l    d  LZMA_DEC30 0 LZMA_DEC30
 0000000000000000 l    d  ELFMAINZ 0 ELFMAINZ
@@ -39,7 +39,7 @@ SYMBOL TABLE:
 RELOCATION RECORDS FOR [ELFMAINX]:
 OFFSET           TYPE              VALUE
 0000000000000000 R_AARCH64_ABS32   MFLG
-0000000000000004 R_AARCH64_CALL26  ELFMAINZ+0x00000000000000e4
+0000000000000004 R_AARCH64_CALL26  ELFMAINZ+0x00000000000000c4
 
 RELOCATION RECORDS FOR [LZMA_ELF00]:
 OFFSET           TYPE              VALUE
@@ -47,4 +47,4 @@ OFFSET           TYPE              VALUE
 
 RELOCATION RECORDS FOR [ELFMAINZ]:
 OFFSET           TYPE              VALUE
-0000000000000134 R_AARCH64_ABS32   O_BINFO
+000000000000010c R_AARCH64_ABS32   O_BINFO