[build] Rename x86 project to "x64"

Build output will now be in build-x64 instead of build-x86
Renamed and updated various scripts as appropriate

Within zircon, $ARCH remains "x86".
This avoids having to rename kernel/arch/x86/ and change a lot of #include paths
Removed unnecessary $SUBARCH build variable

Renamed the kernel target from pc-x86 to pc, and the board name in boards.list
from "x86" to "pc".

Change-Id: I0eb44a21ba731653b92f3ce2b78a7819c0a8f61c

[ulib][musl] Optimized aarch64 string routines

These are taken verbatim from Linaro's cortex-strings package.
One mechanical cosmetic change to the assembly was necessary for
it to build with Clang/LLVM's assembler because the Linaro code
was using a GAS variant of the official syntax instead of what
the ARMv8 manual says to use. We can revert that to go back
to verbatim when https://bugs.llvm.org//show_bug.cgi?id=34488 is fixed.

MG-1111 #done

Change-Id: I186ee61b9184f247ba77a27622389eee4bde493c

[ulib][musl] Don't use x86-64 assembly version of mempcpy in ASan build

The ASan runtime intercepts memcpy to provide checking on its pointer
arguments, but expects libc's mempcpy to do its own checking. So use
the vanilla C version that will just call the intercepted memcpy.

Change-Id: I06b3d46410d2ea85916b9e8de3507f6e577d1018

[musl] Remove the extended locale functions

The C library provides locale aware variants of several ctype.h and
string.h functions, but all these functions completely ignore the
locale setting and simply call the non-locale variant. Rather than
pretending that our C library supports locale, we should omit these
functions for now and if needed, actually implement them in the
future with the proper locale support.

Change-Id: Ibc7eeda464232340c2dc78d63331b6b02b1b7341

[ulib][musl] Rewrite x86 memcpy, mempcpy, and memset

On recent Intel chips (ERMS), 'rep movsb' and 'rep stosb'
by themselves are better than any of the shenanigans the
old musl assembly code was doing.

Change-Id: I1f50dd55aaf9b4a526fd70c64254766fc0498a2d

[ulib][musl] rename .s files to .S

This aligns with the rest of Magenta and allows for some build
system cleanup and simplification.

Change-Id: I2dfadccf3eaf875c3b27ed22454588e0b1fa2037

[musl] Remove arm32 target from libc

Change-Id: I3a1fb40b6a3abe4522fd0799f1319ac2d974aefe

[build] build shared libraries and separate crt1.o

- shared libraries are a work in progress, this change provides
the core build machinery to support them
- user library modules that declare a MODULE_SONAME cause a
shared library to be built
- user apps and libs may use MODULE_LIBS to depend on shared
libs and MODULE_STATIC_LIBS to depend on static libs
- MODULE_DEPS is currently converted to MODULE_STATIC_LIBS
- libc is arranged to build a separate shared version but
that is disabled at the moment due to build error
- all of userspace is being build -fPIC for the moment
- libruntime and libmagenta are configured to generate shared
libraries
- make the x86 -fno-pic only apply to KERNEL_COMPILEFLAGS

Change-Id: Iccebe078ad18069a97173228d1476a5e95a2b879

[magenta] Initial commit