build: allow to build riscv64 using Makefile

PR-URL: #39048
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

Makefile

@@ -847,6 +847,9 @@ else
 ifeq ($(findstring powerpc,$(shell uname -p)),powerpc)
 DESTCPU ?= ppc64
 else
+ifeq ($(findstring riscv64,$(UNAME_M)),riscv64)
+DESTCPU ?= riscv64
+else
 DESTCPU ?= x86
 endif
 endif
@@ -857,6 +860,7 @@ endif
 endif
 endif
 endif
+endif
 ifeq ($(DESTCPU),x64)
 ARCH=x64
 else
@@ -878,6 +882,9 @@ else
 ifeq ($(DESTCPU),s390x)
 ARCH=s390x
 else
+ifeq ($(DESTCPU),riscv64)
+ARCH=riscv64
+else
 ARCH=x86
 endif
 endif
@@ -886,6 +893,7 @@ 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.