FreeBSD/arm64 MACHINE/MACHINE_ARCH identification

[Ed Maste]

The FreeBSD/arm64 work in progress currently reports "arm64" for the
machine and processor type - i.e., uname -m and uname -p.

It seems that the official, awkward name aarch64 is broadly used
elsewhere - for example, in toolchain triples and autoconf tests.  To
save us grief in the future I think it is worth following suit:

diff --git a/sys/arm64/include/param.h b/sys/arm64/include/param.h
index 5cd0445..525a0e7 100644
--- a/sys/arm64/include/param.h
+++ b/sys/arm64/include/param.h
@@ -43,10 +43,10 @@
 #define STACKALIGN(p)  ((uint64_t)(p) & ~STACKALIGNBYTES)

 #ifndef MACHINE
-#define        MACHINE         "arm64"
+#define        MACHINE         "aarch64"
 #endif
 #ifndef MACHINE_ARCH
-#define        MACHINE_ARCH    "arm64"
+#define        MACHINE_ARCH    "aarch64"
 #endif

I'm not proposing that we rename any of the source files.  I believe
this approach is consistent with the Debian project - they call it the
"arm64" port, but report aarch64 from uname.

I believe it will be much easier for us to carry around any
special-case s/aarch64/arm64/ in the base system (if necessary) than
trying to teach third-party software that the FreeBSD 64-bit ARM
architecture is called arm64 instead of aarch64.

Any objections or concerns?