Fix symbol name mangling for LLVM 7.0.
Fix symbol name mangling for LLVM 7.0.

The Mach-O executable format, used on macOS/Darwin, stores function
names in mangled form, so we need to mangle it ourselves during
function pointer lookup.

Bug b/115344057

Change-Id: I8de5756c52b5af666826134fca8274b4467fa85a
Reviewed-on: https://swiftshader-review.googlesource.com/c/22188
Tested-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Reviewed-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>

google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader