arm64: fix build error in sigcontext.h

I'm seeing this build failure for arm64:

    CC [M]  Documentation/filesystems/configfs/configfs_example_macros.o
  In file included from /usr/include/bits/sigcontext.h:27:0,
                   from /usr/include/signal.h:340,
                   from /usr/include/sys/wait.h:30,
                   from Documentation/accounting/getdelays.c:24:
  .../linux/usr/include/asm/sigcontext.h:61:2: error: unknown type name ‘u64’
    u64 esr;
    ^
  make[2]: *** [Documentation/accounting/getdelays] Error 1

This was introduced by commit 15af1942

:

  arm64: Expose ESR_EL1 information to user when SIGSEGV/SIGBUS

Using __u64 instead of u64 fixes the problem.

Signed-off-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>