build: uname -m is amd64 on freebsd, not x86_64

Fixes: nodejs#13150

Author: bnoordhuis

Makefile

@@ -806,6 +806,9 @@ UNAME_M=$(shell uname -m)
 ifeq ($(findstring x86_64,$(UNAME_M)),x86_64)
 DESTCPU ?= x64
 else
+ifeq ($(findstring amd64,$(UNAME_M)),amd64)
+DESTCPU ?= x64
+else
 ifeq ($(findstring ppc64,$(UNAME_M)),ppc64)
 DESTCPU ?= ppc64
 else
@@ -865,6 +868,7 @@ endif
 endif
 endif
 endif
+endif
 
 # node and v8 use different arch names (e.g. node 'x86' vs v8 'ia32').
 # pass the proper v8 arch name to $V8_ARCH based on user-specified $DESTCPU.