field: Remove x86_64 asm

Widely available versions of GCC and Clang beat our field asm on -O2.
In particular, GCC 10.5.0, which is Bitcoin Core's current compiler
for official x86_64 builds, produces code that is > 20% faster for
fe_mul and > 10% faster for signature verification (see bitcoin#726).

These are the alternatives to this PR:

We could replace our current asm with the fastest compiler output
that we can find. This is potentially faster, but it has multiple
drawbacks:
 - It's more coding work because it needs detailed benchmarks (e.g.,
   with many compiler/options).
 - It's more review work because we need to deal with inline asm
   (including clobbers etc.) and there's a lack of experts reviewers
   in this area.
 - It's not unlikely that we'll fall behind again in a few compiler
   versions, and then we have to deal with this again, i.e., redo the
   benchmarks. Given our history here, I doubt that we'll revolve
   this timely.

We could change the default of the asm build option to off. But this
will also disable the scalar asm, which is still faster.

We could split the build option into two separate options for field
and scalar asm and only disable the field asm by default. But this
adds complexity to the build and to the test matrix.

My conclusion is that this PR gets the low-hanging fruit in terms of
performance. It simplifies our code significantly. It's clearly an
improvement, and it's very easy to review. Whether re-introducing
better asm (whether from a compiler or from CryptOpt) is worth the
hassle can be evaluated separately, and should not hold up this
improvement.

Solves bitcoin#726.

Makefile.am

@@ -37,7 +37,6 @@ noinst_HEADERS += src/field_10x26_impl.h
 noinst_HEADERS += src/field_5x52.h
 noinst_HEADERS += src/field_5x52_impl.h
 noinst_HEADERS += src/field_5x52_int128_impl.h
-noinst_HEADERS += src/field_5x52_asm_impl.h
 noinst_HEADERS += src/modinv32.h
 noinst_HEADERS += src/modinv32_impl.h
 noinst_HEADERS += src/modinv64.h

README.md

@@ -33,7 +33,7 @@ Implementation details
   * Expose only higher level interfaces to minimize the API surface and improve application security. ("Be difficult to use insecurely.")
 * Field operations
   * Optimized implementation of arithmetic modulo the curve's field size (2^256 - 0x1000003D1).
-    * Using 5 52-bit limbs (including hand-optimized assembly for x86_64, by Diederik Huys).
+    * Using 5 52-bit limbs
     * Using 10 26-bit limbs (including hand-optimized assembly for 32-bit ARM, by Wladimir J. van der Laan).
       * This is an experimental feature that has not received enough scrutiny to satisfy the standard of quality of this library but is made available for testing and review by the community.
 * Scalar operations