make: rm LLVM_BUILD_ENV

using this to turn off the frame pointers isn't needed, and doesn't
actually work right now because the reference to it omits the target
triple
make: stop disabling frame pointer elimination

We currently have no need for the frame pointers on any platform. They
may eventually be needed on platforms without an equivalent to the DWARF
call frame information to walk the stack in the garbage collector.

Closes #7477
auto merge of #8695 : thestinger/rust/build, r=pcwalton

We currently have no need for the frame pointers on any platform. They
may eventually be needed on platforms without an equivalent to the DWARF
call frame information to walk the stack in the garbage collector.

Closes #7477