toolchain: Select USE_SSTRIP with external musl toolchain

toolchain: Select USE_SSTRIP with external musl toolchain

When we use the internal toolchain USE_SSTRIP will be selected by
default for musl libc and USE_STRIP when glibc is used. Do the same when
an external toolchain is used. USE_GLIBC will also be set for external
toolchain builds based on the EXTERNAL_TOOLCHAIN_LIBC_USE_GLIBC setting.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 9403810c020cca136149973a3929bf77a1f501aa)