[arch][arm-m] optimize the arch_interrupt_save routine a bit for cortex-m

From looking at the dissassembly the compiler wasn't doing a good job
with the overly complicated version shared with the larger arm cores.
Simplify it by recognizing that you can save the state directly from
PRIMASK and restore it more simply.

arch/arm/include/arch/spinlock.h

@@ -50,6 +50,8 @@ static inline void arch_spin_unlock(spin_lock_t *lock) {
 
 #endif
 
+#if !ARM_ISA_ARMV7M
+
 /* ARM specific flags */
 #define SPIN_LOCK_FLAG_IRQ                      0x40000000
 #define SPIN_LOCK_FLAG_FIQ                      0x80000000 /* Do not use unless IRQs are already disabled */
@@ -86,4 +88,40 @@ arch_interrupt_restore(spin_lock_saved_state_t old_state, spin_lock_save_flags_t
         arch_enable_ints();
 }
 
+#else
+
+/*
+ * slightly more optimized version of the interrupt save/restore bits for cortex-m
+ * processors.
+ */
+
+/* arm-m flags are mostly meaningless */
+#define ARCH_INTERRUPT_SAVE_IRQ                 1
+
+#define ARCH_DEFAULT_SPIN_LOCK_FLAG_INTERRUPTS  ARCH_INTERRUPT_SAVE_IRQ
+
+static inline void
+arch_interrupt_save(spin_lock_saved_state_t *statep, spin_lock_save_flags_t flags) {
+    unsigned int state = 0;
+
+    if (flags == ARCH_INTERRUPT_SAVE_IRQ) {
+        __asm__ volatile("mrs %0, primask" : "=r"(state));
+        /* always disable ints, may be faster than testing and branching around it */
+        arch_disable_ints();
+    }
+    *statep = state;
+}
+
+static inline void
+arch_interrupt_restore(spin_lock_saved_state_t old_state, spin_lock_save_flags_t flags) {
+    /* test the PRIMASK's one bit */
+    if (flags == ARCH_INTERRUPT_SAVE_IRQ) {
+        if ((old_state & 0x1) == 0) {
+            arch_enable_ints();
+        }
+    }
+}
+
+#endif
+
 __END_CDECLS