riscv: Support building with Clang

The -ffixed-gp option of GCC has an exact equivalent of -ffixed-x3 in
Clang.

Signed-off-by: kleines Filmröllchen <filmroellchen@serenityos.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/riscv/config.mk b/arch/riscv/config.mk
index 9cf2aef..9f16dda 100644
--- a/arch/riscv/config.mk
+++ b/arch/riscv/config.mk
@@ -25,7 +25,7 @@
 PLATFORM_ELFFLAGS	+= -B riscv -O elf64-littleriscv
 endif
 
-PLATFORM_CPPFLAGS	+= -ffixed-gp -fpic
+PLATFORM_CPPFLAGS	+= -ffixed-x3 -fpic
 PLATFORM_RELFLAGS	+= -fno-common -ffunction-sections -fdata-sections
 LDFLAGS_u-boot		+= --gc-sections -static -pie