Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

libc/amd64: Do not export .cerror.

For some reason, libc exports the symbol .cerror (HIDENAME(cerror)), albeit
in the FBSDprivate_1.0 version. It looks like there is no reason for this
since it is not used from other libraries. Given that it cannot be accessed
from C and its strange calling convention, it is rather unlikely that other
things rely on it. Perhaps it is from a time when symbols could not be
hidden.

Most of the amd64 assembler code jumps to .cerror using the GOT. It can jump
to it directly now, as in non-PIC mode.

There are also some minor size optimizations to instructions but they yield
virtually no benefit in the size of libc.so.7 due to padding.

Reviewed by: kib

Add section .note.GNU-stack for assembly files used by 386 and amd64.

We've been lax about matching END() macros in asm code for some time. This
is used to set the ELF size attribute for functions. It isn't normally
critical but some things can make use of it (gdb for stack traces).
Valgrind needs it so I'm adding it in. The problem is present on all
branches and on both i386 and amd64.

Sigh. I can't win anything. Use addq rather than addl with %rsp.

Apply same basic fix for getcontext(2) as for i386. Store the return
value for getcontext() in a preserved register rather than on the stack.
The second time around, the stack value would likely have changed so we
can't depend on it for the return value.