valgrind support for glibc master

[Florian Weimer <fweimer at redhat dot com>]

glibc performs a quick test run using valgrind as part of the build process.

Lately, this started crashing:

+ elf/ld.so --library-path .:elf:nptl:dlfcn /usr/bin/valgrind elf/ld.so
--library-path .:elf:nptl:dlfcn /usr/bin/true
==924== Memcheck, a memory error detector
==924== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==924== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==924== Command: elf/ld.so --library-path .:elf:nptl:dlfcn /usr/bin/true
==924==
ARM64 front end: branch_etc
disInstr(arm64): unhandled instruction 0xD5380000
disInstr(arm64): 1101'0101 0011'1000 0000'0000 0000'0000
==924== valgrind: Unrecognised instruction at address 0x11f548.
==924==    at 0x11F548: init_cpu_features (cpu-features.c:32)
==924==    by 0x11F548: dl_platform_init (dl-machine.h:241)
==924==    by 0x11F548: _dl_sysdep_start (dl-sysdep.c:231)
==924==    by 0x10981B: _dl_start_final (rtld.c:412)
==924==    by 0x109AAB: _dl_start (rtld.c:520)
==924==    by 0x108F47: ??? (in
/builddir/build/BUILD/glibc-2.25-545-g9649350/build-aarch64-redhat-linux/elf/ld.so)

The line in question is:

      asm volatile ("mrs %0, midr_el1" : "=r"(id));

That seems to match the instruction bit pattern, too.  There is a check
around it:

  if (hwcap & HWCAP_CPUID)
    {
      register uint64_t id = 0;
      asm volatile ("mrs %0, midr_el1" : "=r"(id));
      cpu_features->midr_el1 = id;
    }
  else
    cpu_features->midr_el1 = 0;

I think this code is fine.  Unfortunately, I don't know if I'll be able
to get a disassembly or debug this any further.  There are a couple of
potential causes (GLRO (dl_hwcap) is not initialized correctly in glibc,
HWCAP_CPUID is not masked by the kernel or valgrind despite the lack of
support, GCC schedule the volatile asm statement before the condition).

Is anyone else seeing this?

I will disable the valgrind sanity test during the Fedora build for the
time being.

Thanks,
Florian