build: recognize amd64 in CMAKE_SYSTEM_PROCESSOR

On FreeBSD, cmake populates `CMAKE_SYSTEM_PROCESSOR` with `uname -p`, which
results in "amd64", not "AMD64".

Author: mhjacobson

CMakeLists.txt

@@ -116,7 +116,7 @@ endif()
 if(SWIFT_HOST_VARIANT_ARCH)
   set(SWIFT_HOST_VARIANT_ARCH_default "${SWIFT_HOST_VARIANT_ARCH}")
 else()
-  if(CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64|AMD64")
+  if(CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64|AMD64|amd64")
     set(SWIFT_HOST_VARIANT_ARCH_default "x86_64")
   elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64|ARM64|arm64")
     if(SWIFT_HOST_VARIANT_SDK_default STREQUAL OSX)