Bring back one combined arch use case.

In the cleanup of the darwin toolset we eliminated the cimbined
architecture option. Until we implement a better combined architecture
model add back the one use case in current demand. That is the macOS
arm64+x86_64 unity build. As such this adds a single "arm+x86"
architecture and corresponding common handling to add the gcc toolset
options for macOS.

src/tools/features/architecture-feature.jam

@@ -47,6 +47,9 @@ feature.feature architecture
 
         # z Systems (aka s390x)
         s390x
+
+        # Combined architecture(s)
+        arm+x86
     :
         propagated optional
     ;

src/tools/gcc.jam

@@ -1272,3 +1272,8 @@ cpu-flags gcc OPTIONS : arm : cortex-r5 : -mcpu=cortex-r5 ;
 cpu-flags gcc OPTIONS : arm : cortex-r5+vfpv3-d16 : -mcpu=cortex-r5 -mfpu=vfpv3-d16 -mfloat-abi=hard ;
 # AIX variant of RS/6000 & PowerPC
 toolset.flags gcc AROPTIONS <address-model>64/<target-os>aix : "-X64" ;
+
+# This is a remporary solution for doing combined architecture builds on macOS.
+toolset.flags gcc
+    OPTIONS <target-os>darwin/<architecture>arm+x86/<address-model>64
+    : "-arch arm64" "-arch x86_64" ;