[ubsan] fix some bugs and warnings discovered by ubsan

- X86 cpuid feature list dump was using the wrong array and walking off
  the end of one.
- GICv2 code had a left shift by up to 31 of an integer. Needs to be
  unsigned.
- PLIC same as GIC code.
- fdtwalker code should be using a bytewise accessor based helper
  function for reading large integers out of an unaliged FDT.
- PCI BIOS32 search code could do a 32bit unaligned read of a string,
  switch to using memcmp.

dev/bus/pci/backend/bios32.cpp

@@ -82,7 +82,7 @@ static pci_bios_info *find_pci_bios_info(void) {
     uint i;
 
     while (head < (uint32_t *) (0x000ffff0 + KERNEL_BASE)) {
-        if (*head == *(uint32_t *) pci_bios_magic) {
+        if (memcmp(head, pci_bios_magic, sizeof(pci_bios_magic)) == 0) {
             // perform the checksum
             sum = 0;
             b = (int8_t *) head;