goarch: Add riscv64

Although RISC-V 64-bit doesn't have official golang support there are
forks that now exist with at least some support and work is ongoing in
the upstream tree. In order to be able to use the goarch class add
support for RISC-V.

For more details see here:
https://github.com/golang/go/issues/27532

(From OE-Core rev: 91e3b2a762124bf5cf923654ef3a7c871b84c82f)

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/goarch.bbclass

@@ -68,6 +68,8 @@ def go_map_arch(a, d):
         return 'ppc64'
     elif re.match('p(pc|owerpc)(64el)', a):
         return 'ppc64le'
+    elif a == 'riscv64':
+        return 'riscv64'
     else:
         raise bb.parse.SkipRecipe("Unsupported CPU architecture: %s" % a)