gn: Don’t use .rsp files; rationalize descriptions and output dirs
gn: Don’t use .rsp files; rationalize descriptions and output dirs

.rsp files were omitted from the Windows build (where they’re probably
more likely to be useful) for simplicity, since mini_chromium and
Crashpad shouldn’t need them. They were left in place for link steps on
other platforms, though. This removes the use of .rsp files, which
should make it easier to debug the build.

Some of the tool descriptions were a little wacky. Some were just plain
wrong, and others were just a little wrong. Rather than trying to match
Chromium step-for-step, this standardizes on a set of meaningful and
recognizable names not likely to be confused for anything else: CC, CXX,
OBJC, OBJCXX, ASM, AR, SOLINK_MODULE, LINK, STAMP, and COPY.

Output directories are also handled more consistently.

The Objective-C steps are defined on macOS only.

Bug: crashpad:79
Change-Id: I1160279f06ccbb663087be494795588f4a637843
Reviewed-on: https://chromium-review.googlesource.com/834228
Reviewed-by: Scott Graham <e134406e1863f3b48b0795831afac5c0595efb36@chromium.org>

chromium/mini_chromium,chromium/mini_chromium,chromium/mini_chromium