riscv: gcov: enable gcov for RISC-V

This patch enables GCOV code coverage measurement on RISC-V.
Lightly tested on QEMU and Hifive Unleashed board, seems to work as
expected.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

Documentation/features/debug/gcov-profile-all/arch-support.txt

@@ -23,7 +23,7 @@
     |    openrisc: | TODO |
     |      parisc: | TODO |
     |     powerpc: |  ok  |
-    |       riscv: | TODO |
+    |       riscv: |  ok  |
     |        s390: |  ok  |
     |          sh: |  ok  |
     |       sparc: | TODO |