Fix build for riscv64 arch

Fixes:
In file included from ../src/base/environment.cc:5:
In file included from ../src/base/environment.h:13:
../src/util/build_config.h:171:2: error: Please add support for your architecture in build_config.h
 #error Please add support for your architecture in build_config.h
 ^
1 error generated.

Change-Id: I140d6ef5daeeceaa46ce2c1888e8a549aa40f8d8
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/11100
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>

src/util/build_config.h

diff --git a/src/util/build_config.h b/src/util/build_config.h
index be9b532..667f009 100644
--- a/src/util/build_config.h
+++ b/src/util/build_config.h
@@ -167,6 +167,9 @@
 #elif defined(__e2k__)
 #define ARCH_CPU_64_BITS 1
 #define ARCH_CPU_LITTLE_ENDIAN 1
+#elif defined(__riscv) && (__riscv_xlen == 64)
+#define ARCH_CPU_64_BITS 1
+#define ARCH_CPU_LITTLE_ENDIAN 1
 #else
 #error Please add support for your architecture in build_config.h
 #endif