[kernel] Unify CPUs' control register settings

Previously, the CPU control registers were being set in a number of
different ways. Now, since the APs' need this to be set in the CPU
initialization code, always do it there. This removes some of the
settings from the bootloader, and some unused ones from smp.s.
Additionally, the control registers' flags are now enums in cpu.h and
manipulated via util::bitset.

src/boot/hardware.cpp

@@ -60,12 +60,7 @@ setup_control_regs()
 {
     uint64_t cr4 = 0;
     asm volatile ( "mov %%cr4, %0" : "=r" (cr4) );
-    cr4 |=
-        0x000080 | // Enable global pages
-        0x000200 | // Enable FXSAVE/FXRSTOR
-        0x010000 | // Enable FSGSBASE
-        0x020000 | // Enable PCIDs
-        0;
+    cr4 |= 0x000080; // Enable global pages
     asm volatile ( "mov %0, %%cr4" :: "r" (cr4) );
 
     // Set up IA32_EFER