tsan: Enable 48-bit VMA support on aarch64

This patch adds 48-bits VMA support for tsan on aarch64.  As current
mappings for aarch64, 48-bit VMA also supports PIE executable.  This
limits the mapping mechanism because the PIE address bits
(usually 0aaaaXXXXXXXX) makes it harder to create a mask/xor value
to include all memory regions.  I think it is possible to create a
large application VAM range by either dropping PIE support or tune
current range.

It also changes slight the way addresses are packed in SyncVar structure:
previously it assumes x86_64 as the maximum VMA range.  Since ID is 14 bits
wide, shifting 48 bits should be ok.

Tested on x86_64, ppc64le and aarch64 (39 and 48 bits VMA).

llvm-svn: 277137