swdptap: Replace popcount with parity

* BMF on Cortex-M3 relies on libgcc2 software builtin impls,
  not the x86-64-v2 popcnt insn, so adjust accordingly (parity is cheaper)
* More type- and const-correctness (bmp_gpio_get/set)

src/platforms/common/swdptap.c

@@ -130,8 +130,8 @@ static bool swdptap_seq_in_parity(uint32_t *ret, size_t clock_cycles)
 	for (volatile uint32_t counter = target_clk_divider + 1; counter > 0; --counter)
 		continue;
 
-	size_t parity = __builtin_popcount(result);
-	parity += gpio_get(SWDIO_IN_PORT, SWDIO_IN_PIN) ? 1U : 0U;
+	const bool parity = __builtin_parity(result);
+	const bool bit = gpio_get(SWDIO_IN_PORT, SWDIO_IN_PIN);
 
 	gpio_set(SWCLK_PORT, SWCLK_PIN);
 	for (volatile uint32_t counter = target_clk_divider + 1; counter > 0; --counter)
@@ -140,7 +140,7 @@ static bool swdptap_seq_in_parity(uint32_t *ret, size_t clock_cycles)
 	*ret = result;
 	/* Terminate the read cycle now */
 	swdptap_turnaround(SWDIO_STATUS_DRIVE);
-	return parity & 1U;
+	return parity != bit;
 }
 
 static void swdptap_seq_out_clk_delay(uint32_t tms_states, size_t clock_cycles) __attribute__((optimize(3)));
@@ -182,9 +182,9 @@ static void swdptap_seq_out(const uint32_t tms_states, const size_t clock_cycles
 
 static void swdptap_seq_out_parity(const uint32_t tms_states, const size_t clock_cycles)
 {
-	int parity = __builtin_popcount(tms_states);
+	const bool parity = __builtin_parity(tms_states);
 	swdptap_seq_out(tms_states, clock_cycles);
-	gpio_set_val(SWDIO_PORT, SWDIO_PIN, parity & 1U);
+	gpio_set_val(SWDIO_PORT, SWDIO_PIN, parity);
 	for (volatile uint32_t counter = target_clk_divider + 1; counter > 0; --counter)
 		continue;
 	gpio_set(SWCLK_PORT, SWCLK_PIN);