[compiler-rt] Allow nm program to be over-ridden for global symbol detection

Summary:
While cross-compiling, a custom nm program may be required. This will also allow for the
use of llvm-nm if desired.

Reviewers: samsonov, beanz, compnerd, eugenis

Subscribers: kubabrecka, dberris, llvm-commits

Differential Revision: https://reviews.llvm.org/D23278

llvm-svn: 278187