Use the AT&T syntax to support old LLVM on rust-lang/rust

adds testing for almost every numerical intrinsic

and add various methods that will be used for improved fuzzing

Should hopefully fix the master branch...

* Use a no-asm feature instead of an asm feature

This works better as core/alloc/std have trouble supporting default
featues in this crate.

Signed-off-by: Joe Richey <joerichey@google.com>

* Have no-asm disable arm assembly intrinsics

Signed-off-by: Joe Richey <joerichey@google.com>

* Reorganize mem functions

This reduces the amount of platform-specific code

Signed-off-by: Joe Richey <joerichey@google.com>

* Use ERMSB implementations if the feature is set

Signed-off-by: Joe Richey <joerichey@google.com>

* Add non-aligned benchmarks

Signed-off-by: Joe Richey <joerichey@google.com>

These are exposed in core::f32

close #354 
c.f. rust-lang/rust#62729
Patch from @whitequark (https://paste.debian.net/1168430/)

* mem: Move mem* functions to separate directory

Signed-off-by: Joe Richey <joerichey@google.com>

* memcpy: Create separate memcpy.rs file

Signed-off-by: Joe Richey <joerichey@google.com>

* benches: Add benchmarks for mem* functions

This allows comparing the "normal" implementations to the
implementations provided by this crate.

Signed-off-by: Joe Richey <joerichey@google.com>

* mem: Add REP MOVSB/STOSB implementations

The assembly generated seems correct:
    https://rust.godbolt.org/z/GGnec8



Signed-off-by: Joe Richey <joerichey@google.com>

* mem: Add documentations for REP string insturctions

Signed-off-by: Joe Richey <joerichey@google.com>

* Use quad-word rep string instructions

Signed-off-by: Joe Richey <joerichey@google.com>

* Prevent panic when compiled in debug mode

Signed-off-by: Joe Richey <joerichey@google.com>

* Add tests for mem* functions

Signed-off-by: Joe Richey <joerichey@google.com>

* Add build/test with the "asm" feature

Signed-off-by: Joe Richey <joerichey@google.com>

* Add byte length to Bencher

Signed-off-by: Joe Richey <joerichey@google.com>

These macros and functions are only in the public interface for testing purposes or because of `#[macro_export]` pollution

Rebenchmarking this showed that perf changed for the worse only on really low end CPUs

Add uefi arch x86 probestack support

add 32 bit shift instructions

This time using the proper procedure for including libm.

0.1.33 was published without the libm submodule

Add compiler-rt fallbacks on aarch64-musl

Fix CI url for compiler-rt source

add mips/mips64 compiler-rt fallbacks so that libgcc is not required

1. In UEFI x86 arch, probestack need triple underscore.

2.  In UEFI, probestack function do things like _chkstk(in MSVC).
    MSVC x32's _chkstk and cygwin/mingw's _alloca adjust %esp themselves
    MSVC x64's __chkstk and cygwin/mingw's ___chkstk_ms do not adjust
    %rsp themselves.
    But current probestack doesn't adjust esp. And LLVM doesn't generate
    sub %eax, %esp after probestack. So we adjust esp in probestack like
    MSVC x32's _chkstk.