fix mixup in previous change to gcc wrapper

make gcc-specific headers (intrinsics, etc.) available with wrapper

they are intentionally listed after the libc include directory so that
the gcc float.h, etc. don't get used in place of the libc ones.

gcc wrapper improvement: leave libgcc dir in the library path

this is needed in case -lgcc is passed explicitly on the link command
line, for example if the wrapper is being used to build musl itself.

make gcc wrapper rewrite link options rather than just extending them

this is not tested yet, but should work to get rid of unwanted
--hash-style=gnu hacks present in some distro-patched gcc versions.

fix musl-gcc wrapper to work with -pie

linking the wrong crt1.o resulted in textrels and thus crashing

new gcc wrapper, entirely specfile based

the _concept_ of this wrapper has been tested extensively, but the
integration with the build/install system, and using a persistent
specfile rather than one generated at build-time, have not been
heavily tested and may need minor tweaks.

this approach should be a lot more robust (and easier to improve) than
writing a shell script that's responsible for trying to mimic gcc's
logic about whether it's compiling or linking, building shared libs or
executable files, etc. it's also lighter weight and should result in
mildly faster builds when using the wrapper.