x86/boot: move some __high_start code and data into init sections

x86/boot: move some __high_start code and data into init sections

Half of __high_start is strictly for the BSP and will only be run once on
boot.  To complement 'start_secondary', create 'start_bsp' and move it into
the init.text section.

The interrupt handler 'ignore_int' is patched into the BSPs IDT, but fully
replaced with real handlers early during boot.  The BSPs IDT is used by APs
until midway through start_secondary, but after the real handlers have been
installed.  Therefore, 'ignore_int' can move to init.text.  Furthermore, its
strings can move to init.rodata.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>