Add GetBase support for kernel mappings as reported by Linux Perf

Linux Perf reports kernel mappings with an offset == start.
Currently GetBase() cannot handle such a case.

This behavior can be seen from a perf.report data with samples in the kernel:

$ sudo perf report -D perf.data | grep MMAP
0 0x108 [0x50]: PERF_RECORD_MMAP -1/0: [0xffffffffaf800000(0x1087a000) @ 0xffffffffaf800000]: x [kernel.kallsyms]_text

Generalize GetBase for shared libraries to handle cases when both process
mapping offset and program header addresses are non-zero.

Based on process mapping information, a sample at a virtual address x maps to
a file offset fx = x - map_start + map_offset.

The program header for the segment with the .text section may have non-zero
mapping information. Thus, a file offset fx maps to a symbol address
sx = fx - ph_offset + ph_virt_addr.

Thus, sx = x - map_start + map_offset - ph_offset + ph_virt_addr, and the base
address for symbolization is map_start - map_offset + ph_offset - ph_virt_addr.

The kernel mapping record has the offset of 0xc000000000000000 on
PowerPC64 along with the same value for the mapping start.  Both values
come from arch/powerpc/Kconfig.  This case is not handled by any of the
conditions in the current getBase() code, so update the existing code
handling the kernel case to handle the PowerPC64 case.

Also update the build instructions to use go get