x86/boot: Improve code generation from bootsym()

x86/boot: Improve code generation from bootsym()

The code generation for bootsym() is atrocious, and unnecessarily complicated.
Given the appropriate physical address, all we need is to construct a virtual
address of the appropriate type.

  add/remove: 0/0 grow/shrink: 0/9 up/down: 0/-4256 (-4256)
  Function                                     old     new   delta
  kexec_reserve_area.constprop                 165     159      -6
  reset_videomode_after_s3                     231      70    -161
  identify_cpu                                1341    1176    -165
  parse_acpi_sleep                             408     240    -168
  early_init_intel                             632     440    -192
  __cpu_up                                    1983    1682    -301
  do_platform_op                              6469    5526    -943
  compat_platform_op                          6433    5482    -951
  __start_xen                                12939   11570   -1369
  Total: Before=3341298, After=3337042, chg -0.13%

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>