Merge #13788: Fix --disable-asm for newer assembly checks/code

4207c1b35c configure: Initialise assembly enable_* variables (Luke Dashjr)
afe0875577 configure: Skip assembly support checks, when assembly is disabled (Luke Dashjr)
d8ab8dc12d configure: Invert --enable-asm help string since default is now enabled (Luke Dashjr)

Pull request description:

  Fixes #13759

  Also inverts the help (so it shows `--disable-asm` like other enabled-by-default options, and initialises the flag variables.

ACKs for commit 4207c1:
  laanwj:
    makes sense, utACK 4207c1b35c
  achow101:
    utACK 4207c1b35c
  ken2812221:
    ACK 4207c1b35c
  practicalswift:
    tACK 4207c1b35c

Tree-SHA512: a30be1008fd8f019db34073f78e90a3c4ad3767d88d7c20ebb83e99c7abc23552f7da3ac8bd20f727405799aff1ecb6044cf869653f8db70478a074d0b877e0a

configure.ac

@@ -195,8 +195,8 @@ AC_ARG_ENABLE([glibc-back-compat],
   [use_glibc_compat=no])
 
 AC_ARG_ENABLE([asm],
-  [AS_HELP_STRING([--enable-asm],
-  [Enable assembly routines (default is yes)])],
+  [AS_HELP_STRING([--disable-asm],
+  [disable assembly routines (enabled by default)])],
   [use_asm=$enableval],
   [use_asm=yes])
 
@@ -339,6 +339,13 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-implicit-fallthrough"],,[[$CXXFLAG_WERROR]])
 fi
 
+enable_hwcrc32=no
+enable_sse41=no
+enable_avx2=no
+enable_shani=no
+
+if test "x$use_asm" = "xyes"; then
+
 # Check for optional instruction set support. Enabling these does _not_ imply that all code will
 # be compiled with them, rather that specific objects/libs may use them after checking for runtime
 # compatibility.
@@ -416,6 +423,8 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
 )
 CXXFLAGS="$TEMP_CXXFLAGS"
 
+fi
+
 CPPFLAGS="$CPPFLAGS -DHAVE_BUILD_INFO -D__STDC_FORMAT_MACROS"
 
 AC_ARG_WITH([utils],