Fix cross-building on Linux/aarch64

Add necessary bits to detect ELF format on Linux/aarch64; note that
Linux calls it aarch64 where we would typically call it arm64 (uname -m)

Reviewed by:	arichardson, emaste, imp
Sponsored by:	Ampere Computing LLC
Sponsored by:	Klara Inc.
Differential Revision:	https://reviews.freebsd.org/D32542

contrib/elftoolchain/common/native-elf-format

@@ -39,6 +39,8 @@ $1 ~ "Machine:" {
             elfarch = "EM_X86_64";
         } else if (match($0, "PowerPC64")) {
             elfarch = "EM_PPC64";
+        } else if (match($0, "AArch64")) {
+            elfarch = "EM_AARCH64";
         } else {
             elfarch = "unknown";
         }

tools/build/Makefile

@@ -125,6 +125,9 @@ _host_arch=${MACHINE}
 .if ${_host_arch} == "x86_64"
 # bmake on Linux/mac often prints that instead of amd64
 _host_arch=amd64
+.elif ${_host_arch} == "aarch64"
+# Linux calls arm64, aarch64, across the board
+_host_arch=arm64
 .elif ${_host_arch:Mppc*}
 _host_arch=powerpc
 .endif