[XRay][compiler-rt] Allow for defining defaults at compile-time

Summary:
Currently, defaulted options cannot be overriden easily. As an example,
we always patch the binary pre-main unless the user defines the option
at the commandline to inhibit this behaviour. This change allows for
building different versions of the XRay archive with different defaults
(whether to patch pre-main, or whether to use FDR mode by default, etc.)
and giving that choice at link-time.

For now we don't create different archives that have different defaults,
but those could be added in later revisions. This change just enables
the possibility.

Reviewers: pelikan, kcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30022

llvm-svn: 295534