riscv64: don't use jalr when we can just say call.

We don't actually care about the length of this jump, and lld will relax
it to a jal when possible anyway. Better to have people copy & paste
call and tail than jal and j.

Test: treehugger
Change-Id: I889044b95fbb5567189a0d6ef31f81df0e0383cd
diff --git a/linker/arch/riscv64/begin.S b/linker/arch/riscv64/begin.S
index f7509c6..21665cb 100644
--- a/linker/arch/riscv64/begin.S
+++ b/linker/arch/riscv64/begin.S
@@ -33,7 +33,7 @@
   .cfi_undefined ra
 
   mv a0, sp
-  jal __linker_init
+  call __linker_init
 
   // __linker_init returns the address of the entry point in the main image.
   jr a0