runtime: print the address where a panic happened

This is not very useful in itself, but makes it possible to detect this
address in the output. See the next commit.

This adds around 50 bytes to each binary (except for AVR and wasm). This
is unfortunate, but I think this feature is quite useful still.
A future enhancement might be to create a build tag for extended panic
information that's not set by default.

src/runtime/arch_tinygoriscv.go

@@ -6,6 +6,8 @@ import "device/riscv"
 
 const deferExtraRegs = 0
 
+const callInstSize = 4 // 8 without relaxation, maybe 4 with relaxation
+
 // RISC-V has a maximum alignment of 16 bytes (both for RV32 and for RV64).
 // Source: https://riscv.org/wp-content/uploads/2015/01/riscv-calling.pdf
 func align(ptr uintptr) uintptr {