Fix /DEBUG:FASTLINK usage
Fix /DEBUG:FASTLINK usage

It was being set as a C++ compile flag, while it's an MSVC linker flag.
The /D was interpreted as a macro definition, which caused an
IntelliSense error.

Also fix the set_linker_flag CMake macro to correctly append the build
configuration.

Bug: b/147735529
Change-Id: I0e4473a07c224b64ad0aeb54390bced087eea3d8
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/46088
Reviewed-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>
Tested-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Kokoro-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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