Move x86_cpu_to_apicid to the __read_mostly section

Pls., consider applying this patch series.
It contains the following changes:
- Adds two new macros DEFINE_EARLY_PER_CPU_READ_MOSTLY() and DECLARE_EARLY_PER_CPU_READ_MOSTLY().
- Adds "read-mostly" qualifier to the following variables in smp.h:
- cpu_sibling_map
- cpu_core_map
- cpu_llc_shared_map
- cpu_llc_id
- cpu_number
- x86_cpu_to_apicid
- x86_bios_cpu_apicid
- x86_cpu_to_logical_apicid

As long as all the variables above are only written during the initialization, this change is meant to prevent the false sharing. More specifically, on vSMP Foundation platform x86_cpu_to_apicid shared the same internode_cache_line with frequently written lapic_events.

describe, to be more specific) the 8 have read_mostly nature (tlb_vector_offset, cpu_loops_per_jiffy, xen_debug_irq, etc.) and 25 are frequently written (irq_stack_union, gdt_page, exception_stacks, idt_desc, etc.). Assuming that the spread of the rest of the per_cpu variables is similar, identifying the read mostly memories will make more sense in terms of long-term code maintenance comparing to identifying frequently written memories.

v5 changes:
- Changed the description of the patch series and patch 2:
added the analysis of the current per-cpu varibles spread between the read-mostly and frequently written groups.

v4 changes:
- Fixed the authors signatures in the patches.

v3 changes:
- Added the missing definitions of DEFINE_EARLY_PER_CPU_READ_MOSTLY() and DECLARE_EARLY_PER_CPU_READ_MOSTLY() macros in the !CONFIG_SMP code path in arch/x86/include/asm/percpu.h.

thanks,
vlad