Fix the ARM architecture on FreeBSD

The 64-bit ARM architecture spelling on FreeBSD is aarch64, not arm64.
This results in a build failure about a missing
`freebsd/arm64/swiftrt.o` while building the runtimes.

Author: etcwilde

utils/swift_build_support/swift_build_support/targets.py

@@ -290,7 +290,7 @@ class StdlibDeploymentTarget(object):
         "riscv64",
         "s390x"])
 
-    FreeBSD = Platform("freebsd", archs=["x86_64", "arm64"])
+    FreeBSD = Platform("freebsd", archs=["x86_64", "aarch64"])
 
     LinuxStatic = Platform('linux-static', sdk_name='LINUX_STATIC', archs=[
         'x86_64',