[llvm-exegesis] Change map address in memory annotation tests

Test failures have been reported by some LLVM developers in regards to
the low value of of the location where the memory is being mapped into
the virtual address space as it causes problems with some default
configurations of vm.mmap_min_addr. This patch sets it to 2^20 (1048576)
to alleviate this issues as most distros seem to use a default value of
65536.