sys-libs/glibc: revert CPPFLAGS fix for cross-compilers
In https://sourceware.org/PR24577 we discovered that riscv
can't really install libc headers as it needs a cross-compiler.
This revert reintroduces riscv bootstrap on crossdev.
To workaround it use the -without-headers option until
it's a crossdev's default.
Bug: https://sourceware.org/PR24577
Bug: https://bugs.gentoo.org/692052
Bug: https://bugs.gentoo.org/686248
Package-Manager: Portage-2.3.74, Repoman-2.3.17
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>