[libc] Add a README.md file to the AOR directory.

This directory contains Arm's contribution of their optimized routines as

initially uploaded to https://reviews.llvm.org/D75355. Going forward, parts

of the implementations from this directory will be moved over to the

appropriate place in the LLVM-libc tree. This will be done over many patches,

all of which will go through the normal LLVM code review practice, and follow

the LLVM-libc implementation standards.

# Example config.mk

#

# Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.

# See https://llvm.org/LICENSE.txt for license information.

# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

# Subprojects to build

# For now, LLVM-libc project will focus only "math" functions.

SUBS = math # string networking

# Target architecture: aarch64, arm or x86_64

# For now, LLVM-libc project will focus on x86_64 only.

ARCH = x86_64

# Compiler for the target

CC = $(CROSS_COMPILE)gcc

CFLAGS = -std=c99 -pipe -O3

CFLAGS += -Wall -Wno-missing-braces

CFLAGS += -Werror=implicit-function-declaration

# Used for test case generator that is executed on the host

HOST_CC = gcc

HOST_CFLAGS = -std=c99 -O2

HOST_CFLAGS += -Wall -Wno-unused-function

# Enable debug info.

HOST_CFLAGS += -g

CFLAGS += -g

# Optimize the shared libraries on aarch64 assuming they fit in 1M.

#CFLAGS_SHARED = -fPIC -mcmodel=tiny

# Use for cross compilation with gcc.

#CROSS_COMPILE = aarch64-none-linux-gnu-

# Use with cross testing.

#EMULATOR = qemu-aarch64-static

#EMULATOR = sh -c 'scp $$1 user@host:/dir && ssh user@host /dir/"$$@"' --

# Additional flags for subprojects.

math-cflags =

math-ldlibs =

math-ulpflags =

math-testflags =

string-cflags =

networking-cflags =

# Use if mpfr is available on the target for ulp error checking.

#math-ldlibs += -lmpfr -lgmp

#math-cflags += -DUSE_MPFR

# Use with gcc.

math-cflags += -frounding-math -fexcess-precision=standard -fno-stack-protector

math-cflags += -ffp-contract=fast -fno-math-errno

# Use with clang.

#math-cflags += -ffp-contract=fast

# Disable vector math code

#math-cflags += -DWANT_VMATH=0

# Disable fenv checks

#math-ulpflags = -q -f

#math-testflags = -nostatus

# Enable assertion checks.

#networking-cflags += -DWANT_ASSERT

# Avoid auto-vectorization of scalar code and unroll loops

networking-cflags += -O2 -fno-tree-vectorize -funroll-loops