Clean up of build script

• Switch from GCC5 to GCC. GCC5 was deprecated has been deprecated since GCC was added in 202305 (https://github.com/tianocore/edk2/commit/757f502a3b350436877102c3043744e021537c19)
• Use the correct environment variable to set GCC prefix on x86 GCC*_{IA32,X64}_PREFIX has always been set from GCC*_BIN environment variable (https://github.com/tianocore/edk2/blob/258f2d1563c366bf73932f80737776baa71b7ab5/BaseTools/Conf/tools_def.template#L97-L100). Also set the prefix to empty (which is the default) since binutils does not provide x86_64-linux-gnu-objcopy. Behavioral wise this should be exact the same as before, but I left this in to make it clear what is being set. This also makes it easier to adapt the PKGBUILD to build on non-x86 archs, in which case the GCC_BIN env needs to be set and the ARM/AARCH64 one might need to be unset depending on the build arch.
• Build the tool only once. AFAICT, the ARCH env is not used during the build of the tools and the whole packages build just fine without the multiple builds.