machine: fix a case of undefined behaviour with SP handling (#245)

The use of `asm` for register aliasing is supported in two different
contexts:
- local variables (including GNU expression statements) where it may
  only be used for specifying registers for input and output operands to
  extended `asm` syntax.

  c.f. https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables

- global variables where it may be used to observe the contents of a
  register.

  c.f. https://gcc.gnu.org/onlinedocs/gcc/Global-Register-Variables.html#Global-Register-Variables

The two options here is to either to hoist the variable out into a
global variable, but then it should not be in a header due to fears of
ODR in case the optimizer does not inline it away, and thus becomes a
bit more tricky.  The alternative that this change actually adopts is to
explicitly use a move to copy the value out via the GNU extended
assembly syntax.

With this change, it is now possible to build the Proxy Kernel
completely with clang/LLVM and link with LLD.  The generated kernel also
runs under SPIKE and behaves as expected in a simple smoke test (without
any executable prints the expected message, and runs a trivial RVV
example).

machine/mtrap.h

@@ -48,9 +48,14 @@ typedef struct {
   volatile uint32_t* plic_s_ie;
 } hls_t;
 
-#define MACHINE_STACK_TOP() ({ \
-  register uintptr_t sp asm ("sp"); \
-  (void*)((sp + RISCV_PGSIZE) & -RISCV_PGSIZE); })
+#define STACK_POINTER() ({            \
+  uintptr_t __sp;                     \
+  __asm__("mv %0, sp" : "=r"(__sp));  \
+  __sp;                               \
+})
+
+#define MACHINE_STACK_TOP()           \
+  ({ (void*)((STACK_POINTER() + RISCV_PGSIZE) & -RISCV_PGSIZE); })
 
 // hart-local storage, at top of stack
 #define HLS() ((hls_t*)(MACHINE_STACK_TOP() - HLS_SIZE))