[libc] use -DLLVM_TARGETS_TO_BUILD for build bots

[nickdesaulniers]

For example, the 32b arm buildbot is building LLVM with support for every default-enabled backed; that's really not necessary, and slows down builds for postsubmit testing unnecessarily.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

For example, the 32b arm buildbot is building LLVM with support for every default-enabled backed; that's really not necessary, and slows down builds for postsubmit testing unnecessarily.

[nickdesaulniers]

This wouldn't matter if we switch to the runtimes build and replace hdrgen.