all: replace ReadRegister with AsmFull inline assembly

This makes AsmFull more powerful (by supporting return values) and
avoids a compiler builtin.

src/device/arm/arm.go

@@ -52,11 +52,10 @@ func Asm(asm string)
 //             "value":  1
 //             "result": &dest,
 //         })
-func AsmFull(asm string, regs map[string]interface{})
-
-// ReadRegister returns the contents of the specified register. The register
-// must be a processor register, reachable with the "mov" instruction.
-func ReadRegister(name string) uintptr
+//
+// You can use {} in the asm string (which expands to a register) to set the
+// return value.
+func AsmFull(asm string, regs map[string]interface{}) uintptr
 
 // Run the following system call (SVCall) with 0 arguments.
 func SVCall0(num uintptr) uintptr

src/device/avr/avr.go

@@ -15,4 +15,7 @@ func Asm(asm string)
 //             "value":  1
 //             "result": &dest,
 //         })
-func AsmFull(asm string, regs map[string]interface{})
+//
+// You can use {} in the asm string (which expands to a register) to set the
+// return value.
+func AsmFull(asm string, regs map[string]interface{}) uintptr

src/device/riscv/riscv.go

@@ -5,6 +5,17 @@ package riscv
 // optimizer.
 func Asm(asm string)
 
-// ReadRegister returns the contents of the specified register. The register
-// must be a processor register, reachable with the "mov" instruction.
-func ReadRegister(name string) uintptr
+// Run the given inline assembly. The code will be marked as having side
+// effects, as it would otherwise be optimized away. The inline assembly string
+// recognizes template values in the form {name}, like so:
+//
+//     arm.AsmFull(
+//         "st {value}, {result}",
+//         map[string]interface{}{
+//             "value":  1
+//             "result": &dest,
+//         })
+//
+// You can use {} in the asm string (which expands to a register) to set the
+// return value.
+func AsmFull(asm string, regs map[string]interface{}) uintptr

src/runtime/arch_arm.go

@@ -15,5 +15,5 @@ func align(ptr uintptr) uintptr {
 }
 
 func getCurrentStackPointer() uintptr {
-	return arm.ReadRegister("sp")
+	return arm.AsmFull("mov {}, sp", nil)
 }

src/runtime/arch_cortexm.go

@@ -17,5 +17,5 @@ func align(ptr uintptr) uintptr {
 }
 
 func getCurrentStackPointer() uintptr {
-	return arm.ReadRegister("sp")
+	return arm.AsmFull("mov {}, sp", nil)
 }

src/runtime/arch_tinygoriscv.go

@@ -15,5 +15,5 @@ func align(ptr uintptr) uintptr {
 }
 
 func getCurrentStackPointer() uintptr {
-	return riscv.ReadRegister("sp")
+	return riscv.AsmFull("mv {}, sp", nil)
 }