_C_LABEL() and _ASM_LABEL() are no longer useful in the "everything
is ELF" world. Eliminate use of them in amd64, arm64, armv7, i386,
macppc, mips64, and sparc64 code.

ok deraadt@ jca@ krw@

Address the clang 13 "changed binding to STB_WEAK" warning on arm64:
add _?ENTRY_NB to asm.h, switch libc's asm bits to the generic
DEFS.h, and use ENTRY_NB as necessary.

Also, give sizes to the hto

Address the clang 13 "changed binding to STB_WEAK" warning on arm64:
add _?ENTRY_NB to asm.h, switch libc's asm bits to the generic
DEFS.h, and use ENTRY_NB as necessary.

Also, give sizes to the htonl/htons symbols and mark them as weak.

ok kettenis@ miod@

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Add retguard macros for arm64 asm and apply them in the straightforward
cases in kernel and libc.
ok deraadt@

Avoid .align 0 here as well. Also fix a .word that should be a .quad.

ok patrick@

Make brk(2) actually work and sync the brk()/sbrk() implementation with arm,
brininging over all the symbol visibility improvements that guenther@ made.

ok drahn@

Add support for AArch64.