lang/gcc12-devel: Switch back to standard bootstrap

Switch back to STANDARD_BOOTSTRAP on i386, amd64 and aarch64 as default
option instead of LTO_BOOTSTRAP: lto bootstrap produces too many
failures on the package builders for those architectures.

Users wanting to use LTO_BOOTSTRAP can still enable the option
manually.

The option stays enabled by default on powerpc, powerpc64 and
powerpc64le, for which I do not have failure reports from package
builders.