toolchain-external: improve glibc support to test availability of RPC

Basically, the BR2_TOOLCHAIN_EXTERNAL_GLIBC option no longer
unconditionally selects BR2_TOOLCHAIN_HAS_NATIVE_RPC since there are
glibc toolchains that don't have RPC support. All the predefined
toolchain profiles are updated to take into account this change: for
the moment, all glibc toolchains that have pre-defined toolchains have
RPC support, but further patches in the series add pre-defined glibc
toolchains that don't have RPC support. In the case of custom glibc
toolchains, a question is asked to the user so that he can say whether
the external glibc toolchain has RPC support or not. The validity of
this configuration option is checked by the new
check_glibc_rpc_feature function in helpers.mk.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>