Fix OpenBLAS 0.3.8 build on darwin (#15041)

* Fix OpenBLAS 0.3.8 build on darwin

See https://github.com/xianyi/OpenBLAS/issues/2431, patched in
https://github.com/xianyi/OpenBLAS/issues/2431 .

* Add references for patchfile

var/spack/repos/builtin/packages/openblas/openblas-0.3.8-darwin.patch

@@ -0,0 +1,23 @@
+diff --git a/c_check b/c_check
+index fbd1838a..e10def2e 100644
+--- a/c_check
++++ b/c_check
+@@ -263,18 +263,6 @@ if ($architecture ne $hostarch) {
+ 
+ $cross = 1 if ($os ne $hostos);
+ 
+-# rework cross suffix and architecture if we are on OSX cross-compiling for ARMV8-based IOS
+-# the initial autodetection will have been confused by the command-line arguments to clang
+-# and the cross-compiler apparently still claims to build for x86_64 in its CC -E output
+-if (($os eq "Darwin") && ($cross_suffix ne "")) {
+-  my $tmpnam = `xcrun --sdk iphoneos --find clang`;
+-  $cross_suffix = substr($tmpnam, 0, rindex($tmpnam, "/")+1 ); 
+-# this should produce something like $cross_suffix="/Applications/Xcode-10.1.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/";
+-  $cross =1;
+-  $architecture = arm64;
+-}
+-
+-
+ 
+ $openmp = "" if $ENV{USE_OPENMP} != 1;
+ 

var/spack/repos/builtin/packages/openblas/package.py

@@ -86,6 +86,10 @@ class Openblas(MakefilePackage):
           sha256='f1b066a4481a50678caeb7656bf3e6764f45619686ac465f257c8017a2dc1ff0',
           when='@0.3.0:0.3.3')
 
+    # Fix https://github.com/xianyi/OpenBLAS/issues/2431
+    # Patch derived from https://github.com/xianyi/OpenBLAS/pull/2424
+    patch('openblas-0.3.8-darwin.patch', when='@0.3.8 platform=darwin')
+
     # Add conditions to f_check to determine the Fujitsu compiler
     patch('openblas_fujitsu.patch', when='%fj')