Use `-march` correctly (#2302)

Selecting tuning flags depends on compilers and architectures. 
Account for this complication which became more apparent w/
proliferation of Arm64 based chips.

Following from: https://maskray.me/blog/2022-08-28-march-mcpu-mtune

Also fix a new --- sensible --- warning.

arbor/morph/place_pwlin.cpp

@@ -146,7 +146,8 @@ place_pwlin::place_pwlin(const arb::morphology& m, const isometry& iso) {
         arb_assert(!segments.empty());
 
         seg_pos.reserve(segments.size()+1);
-        seg_pos = {0};
+        seg_pos.clear();
+        seg_pos.push_back(0);
         for (auto& seg: segments) {
             seg_pos.push_back(seg_pos.back()+distance(seg.prox, seg.dist));
         }

cmake/CompilerOptions.cmake

@@ -110,18 +110,27 @@ function(set_arch_target optvar optvar_cuda_guarded arch)
         endforeach(line)
         string(REGEX REPLACE "-.*" "" target_model "${target}")
 
-        # Use -mcpu for all supported targets _except_ for x86 and Apple arm64, where it should be -march.
-
+        # Figure out which flags to pass to compiler to tune for concrete
+        # architecture.
+        # See clang / gcc manuals and:
+        # https://maskray.me/blog/2022-08-28-march-mcpu-mtune
         if (CMAKE_CXX_COMPILER_ID MATCHES "AppleClang" AND CMAKE_CXX_COMPILER_VERSION LESS 15)
             set(arch_opt "")
-        else()
-            if("${target}" MATCHES "aarch64-apple-darwin" OR "${target}" MATCHES "arm64-apple-darwin")
-                set(arch_opt "-march=${arch} -mtune=${arch}")
-            elseif(target_model MATCHES "x86|i[3456]86" OR target_model MATCHES "amd64" OR target_model MATCHES "aarch64")
-                set(arch_opt "-march=${arch} -mtune=${arch}")
+        elseif (CMAKE_CXX_COMPILER_ID MATCHES "GNU")
+            if ("${target}" MATCHES "(arm64|aarch64)-.*")
+                # on AArch64, this is correct, ...
+                set(arch_opt "-mcpu=${arch} -mtune=${arch}")
             else()
-                set(arch_opt "-mcpu=${arch}")
-            endif()
+                # ... however on x86 mcpu _is_ mtune _and_ deprecated (since 2003!), but ...
+                set(arch_opt "-march=${arch}")
+            endif ()
+        elseif (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+            # ... clang likes march (and possibly mtune)
+            # See https://discourse.llvm.org/t/when-to-use-mcpu-versus-march/47953/9
+            set(arch_opt "-march=${arch} -mtune=${arch}")
+        else ()
+            message(STATUS "Falling back to -march=${arch} for compiler ${CMAKE_CXX_COMPILER_ID}")
+            set(arch_opt "-march=${arch}")
         endif()
     endif()