[libc] Move DTP_OFFSET definition to pthread_arch.h

All other declarations like this are under arch/. As it was, it was
rather mysterious, as nothing else ever defined it (our supported
architectures all have 0; upstream musl supports mips and ppc where
this is 0x8000).

Test: no functional change
Change-Id: I5f9caed55c45f5a3bdca4a8ad20c1fc7625e7813

[musl] Remove unused MC_PC definitions

Change-Id: I95db711e44510a00fd95937e9f85ec829c980174

[ulib][musl] Make stack-guard and unsafe-sp thread-pointer offsets part of the ABI

This fixes offsets from the thread pointer to the stack-guard
(-fstack-protector canary value) and unsafe-sp (-fsanitize=safe-stack
unsafe stack pointer) words as part of the Fuchsia C/C++ ABI. The
offsets are published in the <magenta/tls.h> header and will be baked
into compilers' target support.

Change-Id: If7f6005554eb94a01accd3c66aaa3c5031fa2e4b

[tls] Reintroduce ELF TLS into musl

Change-Id: I87a7458387aa65b9d1534e51ad5f3abed8587af6

[musl] Use #pragma once in musl's internal headers

This only replaces existing ifdef guards in internal headers. It doesn't
add them to files that didn't have guards, or to public headers.

Change-Id: Ie012ac77ed95ff476291c7391230af086ad8a019

[musl] clang format musl

Change-Id: I3bc6ab7655410514691cd07f08454f3afd41d9b3

[magenta] Initial commit