Add riscv64 support for base gn build

Signed-off-by: Rebecca Chang Swee Fun <rebecca.chang@starfivetech.com>
Change-Id: I89f1f2316b871eb950f974db85c20d9af63e884e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3855657


Reviewed-by: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1040968}