dl: Use "adr" assembler command to get proper load address

This change is a partial revert of commit
66a613c5173456fe0edfa1a89147381d2802d4e4 which imposed usage of
__ehdr_start linker variable to get the address of loaded program.

The elf_machine_load_address() function is declared in the
sysdeps/arm/dl-machine.h header. It is called from _dl_start() entry
point for the program. It shall return the load address of the dynamic
linker program.
With this revert the 'adr' assembler instruction is used instead of a
place holder:

arm-poky-linux-gnueabi-objdump -t ld-linux-armhf.so.3 | grep ehdr
00000000 l       .note.gnu.build-id     00000000      __ehdr_start

which shall be pre-set by binutils.

This is crucial in the QEMU ARM environment for which (when /sbin/init
is executed) values set in __ehdr_start symbol are wrong. This causes
the program to crash very early - when the /lib/ld-linux-armhf.so.3 is
executed as a prerequisite to /sbin/init execution.
The kernel's fs/binfmt_elf.c is though responsible for setting
up execution environment, not binutils.

It looks like the only robust way to obtain the _dl_start offset is to
use assembler instruction - not rely on values provided by binutils.

HW:
Hardware name: ARM-Versatile Express (Run with QEMU)
Tested (affected) kernels v5.1.12, v5.10.62 and v5.14.1

When the /sbin/init is setup for run from Linux kernel's very small
environment with LD_DEBUG=all the __ehdr_start is not shown at all.

Fixes: BZ #28293

sysdeps/arm/dl-machine.h

@@ -38,11 +38,33 @@ elf_machine_matches_host (const Elf32_Ehdr *ehdr)
 }
 
 /* Return the run-time load address of the shared object.  */
-static inline ElfW(Addr) __attribute__ ((unused))
+static inline Elf32_Addr __attribute__ ((unused))
 elf_machine_load_address (void)
 {
-  extern const ElfW(Ehdr) __ehdr_start attribute_hidden;
-  return (ElfW(Addr)) &__ehdr_start;
+  Elf32_Addr pcrel_addr;
+#ifdef SHARED
+  extern Elf32_Addr __dl_start (void *) asm ("_dl_start");
+  Elf32_Addr got_addr = (Elf32_Addr) &__dl_start;
+  asm ("adr %0, _dl_start" : "=r" (pcrel_addr));
+#else
+  extern Elf32_Addr __dl_relocate_static_pie (void *)
+    asm ("_dl_relocate_static_pie") attribute_hidden;
+  Elf32_Addr got_addr = (Elf32_Addr) &__dl_relocate_static_pie;
+  asm ("adr %0, _dl_relocate_static_pie" : "=r" (pcrel_addr));
+#endif
+#ifdef __thumb__
+  /* Clear the low bit of the function address.
+
+     NOTE: got_addr is from GOT table whose lsb is always set by linker if it's
+     Thumb function address.  PCREL_ADDR comes from PC-relative calculation
+     which will finish during assembling.  GAS assembler before the fix for
+     PR gas/21458 was not setting the lsb but does after that.  Always do the
+     strip for both, so the code works with various combinations of glibc and
+     Binutils.  */
+  got_addr &= ~(Elf32_Addr) 1;
+  pcrel_addr &= ~(Elf32_Addr) 1;
+#endif
+  return pcrel_addr - got_addr;
 }
 
 /* Return the link-time address of _DYNAMIC.  */