[compiler-rt] Allow trampoline allocation further and 1 gig.

Summary:
The trampoline allocation limits the memory scanning to 1 gig.
There is an unittest that is allocating a large object which make
it impossible to the trampoline allocator to find a free spot.

see shadow_mapping_failures:
```
char bigchunk[1 << 30];
```

This patch is not fixing the unittest but it's fixing it's infinite
loop behavior.

Reviewers: rnk

Subscribers: llvm-commits, wang0109, chrisha

Differential Revision: https://reviews.llvm.org/D22471

llvm-svn: 275887