toolchain-external: enable OpenMP for supported toolchains

toolchain-external: enable OpenMP for supported toolchains

Enable OpenMP support in the following external toolchains:

toolchain-external-arm-aarch64-be
toolchain-external-arm-aarch64
toolchain-external-arm-arm
toolchain-external-codescape-img-mips
toolchain-external-codescape-mti-mips
toolchain-external-codesourcery-amd64
toolchain-external-codesourcery-mips
toolchain-external-linaro-aarch64-be
toolchain-external-linaro-aarch64
toolchain-external-linaro-arm
toolchain-external-linaro-armeb

Signed-off-by: Ed Blake <ed.blake@sondrel.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>